File MIME Type Checker

About File MIME Type Checker

Choose any file and this tool compares the type your browser claims, which is guessed from the file extension, against the type identified by reading the file's leading bytes, known as magic numbers. A mismatch is a quick way to spot a renamed file, a wrong upload or a document that is not what its name says.

How to use it

1. Click the drop area, or drop a file onto it, to select a file.
2. Wait for the Analyzing message to clear. The file is read in your browser.
3. Look at the status: MIME type confirmed, or MIME mismatch detected.
4. Compare Detected MIME with Browser MIME in the File Details list.
5. Use the Extension badge to see the extension the detected format normally uses.

A JPEG renamed to look like a PNG

holiday.png, which is really JPEG data with the wrong extension

MIME mismatch detected. Detected MIME: image/jpeg. Browser MIME: image/png. Extension badge: jpg.

When to use it

• Checking a download that will not open to see whether it is actually a different format.
• Confirming that files collected for an upload form really are the images or documents their names suggest.
• Debugging a server that rejects a file because its Content-Type header disagrees with the contents.
• Reviewing an unfamiliar attachment before deciding how to open it safely.

Questions about File MIME Type Checker

Is my file uploaded to a server?

No. The file is read into memory in your browser and matched against signatures by a library running on the page. Nothing is transmitted, and closing the tab discards it.

What does unknown (no magic bytes matched) mean?

The file does not start with any signature the detector knows. This is normal for plain text, CSV, JSON, source code and other formats that have no fixed header.

Why does the browser MIME show unknown?

Browsers derive the type from the extension using a small built-in table. Uncommon or missing extensions produce an empty type, which this tool displays as unknown.

Does a matching result prove the file is safe?

No. It only shows that the header and the claimed type agree. A file with a correct header can still contain harmful content, so keep normal scanning and caution in place.

Which formats can it identify?

It recognizes many binary formats by signature, including common images, audio, video, archives, PDF and office documents. Text-based formats without a header are not detectable this way.

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