Intentionally Corrupt — Matroska Bytes With an .mp4 Extension
An intentionally corrupt fixture of a different kind: the bytes are a perfectly valid Matroska file, and the extension says `.mp4`. Nothing is damaged — the file simply lies about what it is. Not a valid MP4 by design. This is what a user's 'converted' file usually turns out to be after a rename, and it separates two kinds of code cleanly. Anything that sniffs the first four bytes finds `1A 45 DF A3`, identifies EBML and plays it. Anything that dispatches on the extension hands it to an MP4 demuxer that immediately fails to find `ftyp`. Note the catalog records this entry's MIME as video/mp4, matching the extension rather than the content — deliberately, because that is precisely the mismatch being reproduced.
mp4
video/mp4
- Integrity
- intentionally corrupt — mislabelled, not damaged
- Actual Format
- Matroska
- Declared Extension
- .mp4
- Magic Bytes
- 1A 45 DF A3 (EBML) — not ftyp
- Playable
- yes, by anything that sniffs content rather than trusting the name
- Bytes
- 12347
Binary mp4: no in-browser preview. Download it above to open in a compatible application.
Specifications
- Integrity
- intentionally corrupt — mislabelled, not damaged
- Actual Format
- Matroska
- Declared Extension
- .mp4
- Magic Bytes
- 1A 45 DF A3 (EBML) — not ftyp
- Playable
- yes, by anything that sniffs content rather than trusting the name
- Bytes
- 12347
Testing contract
Expected to fail- Scenario
- Read the first four bytes, then dispatch the file to a demuxer by extension and by sniffed content in turn.
- Expected result
- The first four bytes are 1A 45 DF A3 (EBML), so the 12,347 bytes are an undamaged Matroska file wearing a .mp4 extension. Content sniffing identifies Matroska and plays it; extension dispatch hands it to an MP4 demuxer which fails to find ftyp. Nothing here is damaged - the file only lies about what it is. The catalog records the MIME as video/mp4 to match the extension rather than the content, deliberately, because that is the mismatch being reproduced.
What is a .mp4 file?
MP4 (MPEG-4 Part 14) is the dominant container for digital video, holding video, audio, subtitle, and metadata tracks in a tree of typed boxes. `ftyp` declares the brand, `moov` carries the sample tables that make seeking possible, and `mdat` holds the media; when an encoder writes `moov` last, playback cannot begin until the file has fully downloaded, which a faststart remux fixes. It derives from Apple's QuickTime format, generalized by ISO into the ISOBMFF base that MOV, 3GP, and HEIF share.
How to use this file
Use an example MP4 to test box parsing and track detection, seeking, range-request streaming, and transcode or thumbnail pipelines: verifying that a file with a trailing `moov` box is still handled, and that codec support is checked per track rather than inferred from the extension.
How to use this file for testing
“Intentionally Corrupt — Matroska Bytes With an .mp4 Extension” is a deterministic Novus Examples fixture for Error handling, Video QA, Video codecs. Deliberately corrupt and invalid files, clearly labelled, for testing how your tool fails.
Documented properties for this file: MP4 · 12,347 bytes. Compare results against paired or grouped companions on this page when present (clean↔damaged, searchable↔scanned, or format twins) so scores stay reproducible across runs.
Download the file once, keep the path stable in CI or local scripts, and treat the spec table as the contract: dimensions, seeds, field lists, and roles are intentional. Corrupt or invalid samples are labelled as such, expect parsers to fail loudly rather than silently accept them.
Media fixtures are short and synthetic by design. Prefer waveform or transcript ground truth in the same group when measuring ASR, trim, upscale, or sync tools; do not assume broadcast-quality masters.
Code examples
<video controls preload="metadata" width="640" src="matroska-renamed-as-mp4.mp4"></video>Related files
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Generated by generation/video_accessibility.py. Free for any use, no attribution required, license.