usefulweb.site File Format Doctor

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File Format Doctor

Find out what a file really is from its bytes, why it will not open and how to convert it.

Analysed in memory and not stored. Max 10 MiB.

Used to compare the extension with the detected format. Ignored when a file is uploaded.

E.g. from `xxd -l 512 file` or `Format-Hex`. At least 16 bytes recommended; 512+ for ZIP-based formats.

Try an example: iPhone photo renamed to .jpg (HEIC inside) · Invoice that is really a Windows program · Word document (ZIP local header with word/ entry)

How it works

Upload a file (or paste its first bytes as hex) and the doctor reads its magic number against a table of 74 formats, looks inside ZIP containers (DOCX, XLSX, ODT, EPUB, JAR, APK), and diagnoses wrong extensions, truncated or encrypted files, text encoding problems and disguised executables — with offline conversion commands.

  • Magic numbers are matched at their documented offsets for 74 formats (e.g. "%PDF-" at 0, "ustar" at 257, "CD001" at 0x8001, "ftyp" brands at 4).
  • ISO-BMFF files are told apart by their ftyp brands (heic/heix → HEIC, mif1 → HEIF, avif → AVIF, qt → QuickTime, M4A → audio, isom/mp42 → MP4).
  • ZIP files are opened by reading the central directory (or local file headers in a sample): [Content_Types].xml + word/ → DOCX, xl/ → XLSX, ppt/ → PPTX, the stored "mimetype" entry → ODT/ODS/ODP/EPUB, AndroidManifest.xml → APK, META-INF/MANIFEST.MF → JAR. General-purpose bit 0 marks encrypted entries.
  • Without a binary signature the bytes are checked for BOMs, UTF-16 patterns, strict UTF-8 validity and typical mojibake sequences (e.g. "é", "’"), then for SVG/HTML/XML/JSON/CSV structure.
  • Truncation checks (uploads only): PDF must end with %%EOF, PNG with IEND, JPEG with FF D9, GIF with 3B, ZIP must have an end-of-central-directory record, RIFF/BMP declared sizes must fit.
  • Conversion suggestions use free, offline tools (LibreOffice, ImageMagick, FFmpeg, 7-Zip, iconv) with the exact command for your file name.

Detection reads bytes only; it does not scan for malware. Never run an executable you did not expect to receive.

Sources

For scripts and AI agents

  • JSON API: POST https://fileformatdoctor.usefulweb.site/api/v1/calculate (same calculation as this page)
  • Interface description: OpenAPI 3.1 · AI guide: llms.txt
  • MCP server (Streamable HTTP): https://fileformatdoctor.usefulweb.site/mcp · tool name file_format_doctor
Example request
curl -s -X POST https://fileformatdoctor.usefulweb.site/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"filename":"IMG_0420.jpg","hexBytes":"00 00 00 18 66 74 79 70 68 65 69 63 00 00 00 00 6d 69 66 31 68 65 69 63"}'

Related tools

  • FileFit – Can this app open, edit or convert that file format? Verified facts with sources.
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