File Type Check

Features

• Compares three things: the file extension, the browser-reported MIME (stated when absent) and the detected magic-byte type
• Detection reuses the same signature table as Magic Bytes — no second copy
• ⚠️ Distinguishes a harmless naming difference from a real mismatch — .zip / application/x-zip-compressed is not flagged
• ⚠️ A real mismatch gets a warning with the consequence: programs that trust the extension will fail
• A missing extension is not a mismatch (no claim is not the same as a wrong claim)
• When the magic bytes are unknown it says so rather than forcing a type
• ZIP-family .docx / .xlsx / .jar are not false-flagged
• Nothing is uploaded; everything runs locally

How to use

1. Drop a file into the area above (or click to choose one)
2. Read the three raw facts: extension, browser-reported MIME, detected type
3. Read the verdict line: consistent, harmless naming difference or mismatch, plus the explanation

FAQ

Why is the extension not enough — why also look at the bytes?

Because the extension is just text that anyone can change: rename a.png to a.jpg and it changes, while not a single byte of the file moves. The magic bytes (the first few) are fixed by the format specification and cannot be renamed away. Most programs trust only the extension, so when the two disagree they read the file wrong. This tool puts the two clues side by side so you see the contradiction before you send or upload the file.

.zip shows as application/x-zip-compressed — should I fix it?

No. That is another spelling of the same format (common on Windows) and does not contradict the content, so the tool calls it a harmless naming difference rather than a warning. Flagging every naming difference would just teach you to ignore the verdict, so the two are deliberately kept apart: .zip as application/x-zip-compressed is fine, a .jpg that is really a PNG is not.

Why is a docx not judged “not a ZIP”?

Because a docx is a ZIP container to begin with (as are xlsx, pptx, jar and apk), so its magic bytes naturally match ZIP. The tool checks whether the extension is among the extensions the detected format declares, so .docx is not false-flagged; the browser reporting the Office type rather than application/zip is normal too and counts as “cannot compare”, not a conflict.