MIME (Multipurpose Internet Mail Extensions) type is a standard label identifying file format in HTTP. Format: type/subtype (e.g., image/png, application/pdf). Browsers use Content-Type headers to decide how to handle files. See our MIME Type Lookup tool for the complete list.

What are magic bytes?

Magic bytes (file signature) are the first few bytes of a file that identify its true format regardless of extension. JPEG always starts with FF D8 FF, PNG with 89 50 4E 47, PDF with %PDF. Security-conscious applications check magic bytes rather than trusting file extensions.

What is the difference between lossy and lossless compression?

Lossy compression permanently removes data to achieve smaller files — JPEG, MP3, H.264. Re-compressing further degrades quality. Lossless compression reduces size without any data loss — PNG, FLAC, ZIP. The original is perfectly reconstructable. Use lossy for distribution, lossless for archival.

What is the difference between a container and a codec?

A codec (H.264, VP9, AAC) encodes/decodes the actual audio or video data. A container (MP4, MKV, WebM) wraps one or more coded streams together with metadata, subtitles, and chapters. The same codec can be in different containers: H.264 in MP4 or MKV.

File Format Glossary — 75+ Terms for Developers

A searchable reference of file format terminology — encoding, compression, image/video/audio concepts, web headers, and security terms. Each entry includes a definition, example, and links to related sample files and tools on TrueFileSize.

75 terms

A

Alpha ChannelImage & Graphics: A fourth channel (beyond RGB) that stores transparency information. 0 = fully transparent, 255 = fully opaque. PNG, WebP, and AVIF support alpha; JPEG does not.Transparent PNG samples→
ASCIIEncoding & Text: American Standard Code for Information Interchange. A 7-bit character encoding covering 128 characters (English letters, digits, symbols). The foundation of most modern encodings.(A = 65, Z = 90, 0 = 48)
Aspect RatioVideo: The ratio of width to height. 16:9 (widescreen), 4:3 (old TV), 9:16 (vertical/TikTok), 1:1 (square/Instagram). Affects how video displays on different screens.

B

Base64Encoding & Text: Binary-to-text encoding that represents binary data as ASCII characters. Increases size by ~33%. Used for embedding images in CSS/HTML, email attachments (MIME), and data URIs.(data:image/png;base64,iVBORw0KGgo...)
Bit Depth (Audio)Audio: Number of bits per audio sample. 16-bit = CD quality (96 dB dynamic range). 24-bit = studio quality (144 dB). Affects dynamic range and noise floor.FLAC bit depths→
BitrateCompression: Amount of data processed per second, measured in bits per second (bps). Higher bitrate = better quality but larger files. MP3: 128-320 kbps. Video: 1-50 Mbps.(MP3 128kbps = ~1 MB/min. Video 5Mbps = ~37 MB/min.)
BOMEncoding & Text: Byte Order Mark. A special Unicode character (U+FEFF) placed at the start of a text file to indicate encoding and byte order. UTF-8 BOM (EF BB BF) causes issues in CSV and JSON parsing.Fix CSV BOM issues→

C

Cache-ControlWeb & HTTP: HTTP header that tells browsers and CDNs how long to store a response. 'max-age=31536000, immutable' = cache for 1 year. 'no-cache' = revalidate every time.
CBRCompression: Constant Bit Rate. The bitrate stays the same throughout the file. Predictable file size but wastes bits on simple sections. Used in streaming.MP3 CBR vs VBR→
CDNWeb & HTTP: Content Delivery Network. A global network of servers that cache and deliver files from the nearest location to the user. Reduces latency and improves download speed.
ChannelsAudio: Number of separate audio signals. Mono = 1. Stereo = 2. 5.1 Surround = 6. Most music is stereo. Podcasts are typically mono.
CharsetEncoding & Text: Character set/encoding declaration. Specifies how bytes map to characters. Always declare UTF-8: <meta charset="UTF-8"> in HTML, Content-Type: text/html; charset=utf-8 in HTTP.
ChecksumGeneral: A fixed-size value computed from file data to verify integrity. If even one bit changes, the checksum differs. MD5 (128-bit), SHA-256 (256-bit). Used to detect corruption.
Chroma SubsamplingImage & Graphics: Reducing color resolution to save space. 4:4:4 = full color. 4:2:0 = half horizontal+vertical color resolution. Used in JPEG, H.264. Barely visible to human eye.
ClippingAudio: Audio distortion that occurs when the signal exceeds the maximum level (0 dBFS). The waveform is 'clipped' flat at the top. Sounds harsh and distorted.
Code PageEncoding & Text: A legacy character encoding table (e.g., CP437, CP1252, Shift-JIS). Before Unicode, different regions used different code pages, causing mojibake when files crossed systems.
CodecCompression: Coder-decoder. Software/hardware that encodes and decodes data. Video codecs: H.264, H.265, VP9, AV1. Audio codecs: MP3, AAC, Opus, FLAC.Video codecs cheat sheet→
Color DepthImage & Graphics: Number of bits used per pixel. 8-bit = 256 colors (GIF). 24-bit = 16.7M colors (JPEG/PNG). 30/36-bit = HDR (10/12-bit per channel).PNG bit depths→
Color GamutImage & Graphics: The range of colors a device or format can represent. sRGB (standard web) < Display P3 (Apple) < Adobe RGB (print) < Rec. 2020 (HDR video).
Compression LevelArchives: A setting that trades compression speed for ratio. Level 1 = fastest, least compression. Level 9 = slowest, best compression. Default is usually 6.
Compression RatioCompression: The ratio of original size to compressed size. 2:1 means 50% reduction. ZIP typically achieves 2-10:1. FLAC: ~2:1. JPEG: 10-20:1.
Container FormatVideo: A wrapper that holds video, audio, subtitle, and metadata tracks. MP4, MKV, WebM, MOV are containers. The container is separate from the codec (H.264, VP9).MKV container→
Content-DispositionWeb & HTTP: HTTP header controlling how the browser handles a response. 'inline' displays in browser. 'attachment; filename="doc.pdf"' forces download with the specified filename.Secure downloads→
Content-TypeWeb & HTTP: HTTP header specifying the MIME type of the response body. Content-Type: image/avif tells the browser to render as AVIF. Missing or wrong Content-Type causes display failures.
CORSWeb & HTTP: Cross-Origin Resource Sharing. A security mechanism that controls which domains can access your resources. Missing CORS headers block fonts, images, and API calls from other origins.

D

Data URIWeb & HTTP: A way to embed file data directly in HTML/CSS using base64 encoding. data:image/png;base64,iVBOR... Avoids an extra HTTP request but increases HTML size by ~33%.
DEFLATECompression: The compression algorithm used by ZIP, gzip, and PNG. Combines LZ77 (dictionary matching) and Huffman coding. Fast and universal.
DPIImage & Graphics: Dots Per Inch. Measures print resolution. 72 DPI for screen display, 300 DPI for professional print. Does not affect pixel dimensions — only physical print size.

E

EndiannessGeneral: The byte order in multi-byte values. Big-endian: most significant byte first (network order). Little-endian: least significant byte first (x86, ARM). Matters for binary file parsing.
Entropy CodingCompression: Compression technique that assigns shorter codes to frequent symbols. Huffman coding and arithmetic coding are common types. Used in JPEG, H.264, ZIP.
ETagWeb & HTTP: Entity Tag. A hash of the file content sent as an HTTP header. Browsers use it for conditional requests (If-None-Match) to avoid re-downloading unchanged files.
EXIFImage & Graphics: Exchangeable Image File Format. Metadata embedded in JPEG/TIFF/HEIC files: camera model, settings, GPS coordinates, timestamp. Privacy risk — should be stripped for web.EXIF privacy risks→

F

File HeaderGeneral: The beginning of a file containing format identification (magic bytes), version info, and structural metadata. Parsers read the header first to understand file structure.
File SignatureGeneral: Another name for magic bytes. The unique byte sequence at the beginning of a file that identifies its format. Tools like 'file' command on Linux use signatures for detection.
Frame RateVideo: Number of images (frames) displayed per second. 24fps = cinema. 30fps = standard. 60fps = smooth gaming/sports. 120fps = slow-motion source.(30fps × 60s = 1800 frames per minute)60fps MP4 sample→

H

HashGeneral: A one-way function that maps data to a fixed-size output. SHA-256, MD5. Used for checksums, password storage, content addressing. Hash of 1GB file = 64 hex characters.
HDRImage & Graphics: High Dynamic Range. Images/video with wider brightness range (10-bit+ per channel). AVIF and HEIC support HDR. Requires compatible display.AVIF HDR support→

I

ICC ProfileImage & Graphics: International Color Consortium profile. Embedded color management data that ensures consistent color across devices. sRGB (web standard) vs Display P3 (Apple/wide gamut).
ID3 TagsAudio: Metadata format for MP3 files storing title, artist, album, year, genre, album art, and lyrics. ID3v1 at file end (128 bytes), ID3v2 at file start (no size limit).MP3 samples→
Interlaced VideoVideo: Legacy technique where each frame contains only odd or even scan lines (fields). Used in broadcast TV. Modern displays use progressive scan. Deinterlacing converts to progressive.
InterlacingImage & Graphics: Loading an image in multiple passes — showing a blurry preview first. PNG uses Adam7 interlacing. JPEG uses progressive mode. Improves perceived load time.

K

KeyframeVideo: A complete frame that doesn't reference other frames. Other frames (P-frames, B-frames) only store differences from the keyframe. More keyframes = better seeking but larger files.

L

Lossless CompressionCompression: Compression that reduces file size without any data loss. The original data is perfectly reconstructable. Used by PNG, FLAC, ZIP, GZIP.FLAC lossless audio→
Lossy CompressionCompression: Compression that permanently removes data to achieve smaller files. Quality degrades with each re-compression. Used by JPEG, MP3, AAC, H.264.Image formats comparison→

M

Magic BytesGeneral: The first few bytes of a file that identify its format regardless of extension. JPEG: FF D8 FF. PNG: 89 50 4E 47. PDF: 25 50 44 46. Used for reliable file type detection.File type validation→
MetadataGeneral: Data about data. File metadata includes: creation date, author, dimensions, codec, GPS coordinates, camera settings. Stored in file headers (EXIF, ID3, XMP).EXIF privacy risks→
MIME SniffingGeneral: When a browser ignores the Content-Type header and guesses the file type from content. Can cause security issues — prevented with X-Content-Type-Options: nosniff header.
MIME TypeWeb & HTTP: Multipurpose Internet Mail Extensions type. A label identifying file format in HTTP headers: application/pdf, image/png, video/mp4. Browsers use it to decide how to handle files.MIME Type Lookup→
MojibakeEncoding & Text: Garbled text caused by reading a file with the wrong character encoding. Example: UTF-8 text read as Latin-1 shows 'Ã©' instead of 'é'.Fix encoding issues→
Multipart ArchiveArchives: A large archive split into multiple smaller files. Common in RAR (.r01, .r02) and ZIP (.z01, .z02). Useful for upload limits and physical media.
MuxingVideo: Multiplexing — combining separate video, audio, and subtitle streams into a single container file. Demuxing is the reverse. ffmpeg -i video.mp4 -i audio.aac -c copy output.mkv.

N

NormalizationAudio: Adjusting audio volume to a target level. Peak normalization sets the loudest point to 0 dBFS. Loudness normalization (LUFS) matches perceived loudness across tracks.
Null ByteGeneral: A byte with value 0x00. In C-strings, it marks the end. In filenames, it was historically used for path traversal attacks (file.php%00.jpg). Modern systems reject null bytes in filenames.

P

PayloadGeneral: The actual content data in a file, as opposed to headers and metadata. In a JPEG, the payload is the compressed image data. In a ZIP, it's the compressed file entries.
PCMAudio: Pulse Code Modulation. Uncompressed digital audio — the raw representation of sound as numbers. WAV and AIFF store PCM data. Largest file size but zero quality loss.WAV samples→
Polyglot FileGeneral: A file that is valid in multiple formats simultaneously. Example: a file that is both a valid JPEG and valid JavaScript. Used in security attacks to bypass content type checks.Polyglot defense→
PPIImage & Graphics: Pixels Per Inch. Screen display density. iPhone 15: 460 PPI. Standard monitors: 96-110 PPI. Retina/HiDPI: 200+ PPI.
Presigned URLWeb & HTTP: A time-limited URL that grants temporary access to a private cloud storage object (S3, R2, GCS). Used for secure file downloads without proxying through your server.S3 presigned URLs→
Progressive ScanVideo: Each frame contains all scan lines (complete image). Standard for all modern displays and web video. 1080p = 1080 progressive. 1080i = 1080 interlaced.

R

Range RequestsWeb & HTTP: HTTP mechanism for downloading partial file content. The client sends 'Range: bytes=0-1024'. The server responds with 206 Partial Content. Essential for video seeking and resumable downloads.
RasterizationImage & Graphics: Converting vector graphics (SVG, font outlines) to pixels (bitmap). Happens at render time in browsers. Opposite of vectorization.
RemuxingVideo: Changing the container without re-encoding (e.g., MKV → MP4). Instant, no quality loss. Only works if the target container supports the codecs.(ffmpeg -i input.mkv -c copy output.mp4)
ResolutionImage & Graphics: The number of pixels in an image, typically expressed as width × height. 1920×1080 = Full HD (2 megapixels). Higher resolution = more detail but larger file.

S

Sample RateAudio: Number of audio samples captured per second. 44,100 Hz = CD quality. 48,000 Hz = video standard. 96,000 Hz = hi-res audio. Higher = better frequency reproduction.WAV sample rates→
Self-Extracting ArchiveArchives: An archive combined with an extraction program (.exe). Double-click to extract without needing separate software. Security risk — often flagged by antivirus.
Solid ArchiveArchives: An archive where all files are compressed as a single stream rather than individually. Better compression ratio but slower random access. 7Z and RAR support solid mode.

T

TarballArchives: A .tar archive — concatenation of files without compression. Usually compressed with gzip (.tar.gz) or bzip2 (.tar.bz2). Standard on Linux/Unix.TAR.GZ samples→
TranscodingVideo: Converting video/audio from one codec to another (e.g., H.264 → H.265). Requires decoding then re-encoding. Slow but necessary for format compatibility.
TransparencyImage & Graphics: The ability to show content behind an image. Requires alpha channel (PNG, WebP, AVIF) or 1-bit mask (GIF). JPEG does not support transparency.

U

UTF-16Encoding & Text: Fixed/variable-width Unicode encoding using 2 or 4 bytes. Used internally by JavaScript, Java, and Windows. Less efficient for ASCII-heavy text than UTF-8.
UTF-8Encoding & Text: Variable-width Unicode encoding that uses 1-4 bytes per character. Backward-compatible with ASCII. The dominant encoding for the web (98%+ of all websites).(English: 1 byte. Chinese: 3 bytes. Emoji: 4 bytes.)

V

VBRCompression: Variable Bit Rate. The bitrate adapts per frame/section — more bits for complex parts, fewer for silence. Better quality-per-byte than CBR but less predictable size.
Vector GraphicsImage & Graphics: Images defined by mathematical shapes (lines, curves, fills) rather than pixels. Scale infinitely without quality loss. SVG is the web standard.SVG samples→

Z

Zip BombArchives: A malicious archive designed to expand to enormous size when extracted. A 42KB file can decompress to 4.5 petabytes. Used for denial-of-service attacks.Zip bomb protection→
Zip SlipArchives: A path traversal vulnerability where a malicious archive contains entries like '../../../etc/passwd', extracting files outside the target directory.Preventing Zip Slip→

File Format Glossary — 75+ Terms for Developers

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Frequently Asked Questions

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