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Check File Type by Bytes

Identify a file’s real format from its magic bytes, see the header values, and catch an extension that does not match the content.

Choose a file

The header is read on this device. Nothing is uploaded.

What the bytes say

Choose a file to identify its format.

The detected format, MIME type and extension check appear here.

Your file is never sent to a server. It is read and processed by your own browser, and the results are not stored.

How to use it

  1. Choose any file or drag it onto the drop area.
  2. Read the format identified from the file’s own bytes.
  3. Check whether the extension agrees, and copy the reported MIME type if you need it.

Limits worth knowing

  • Only the first 64 KB are read. A format whose signature sits later in the file, such as some raw disk images, is not identified.
  • Text formats are recognised heuristically: a JSON or HTML file is named from its opening characters, and an unrecognised UTF-8 file is reported as plain text.
  • The signature identifies a container, not the contents. A well-formed ZIP, PDF or image can still be malicious.
  • An extension this tool does not know is not called a mismatch, because that is not evidence the name is wrong.

Why the bytes matter more than the name

Every operating system decides how to open a file mostly from its extension. That is a convenience, not a property of the file. The header is the only place the format actually lives, so a photo renamed to .pdf still begins with JPEG bytes, and an executable renamed to .jpg still begins with “MZ”.

Portals and upload forms often reject a file because the extension and the detected MIME type disagree. Checking the header before uploading is faster than discovering the mismatch after a rejected submission.

What the header report includes

The tool names the family (image, document, archive, audio, video, font, executable, text), gives the MIME type, and lists the extensions that legitimately belong to that format. ZIP containers are opened far enough to distinguish DOCX, XLSX, PPTX, EPUB and JAR, whose entry names are stored unencrypted in the archive directory.

For a file with no recognisable signature and no readable text, the honest answer is “unrecognised binary”, not a guess. That is reported as such.

Frequently asked questions

Does Check File Type by Bytes upload my files to any server?

No. Check File Type by Bytes runs in your browser, and the file is read there rather than sent to a server for processing. Our host still sees ordinary page-request metadata, which is described on the privacy page.

Is Check File Type by Bytes free to use?

Yes, Check File Type by Bytes is free, with no paid tier, no account, and no watermark added to your output. The limits are technical rather than commercial: file size, page count and supported formats are listed on this page.

What file formats and sizes are supported?

Check File Type by Bytes supports standard files files up to 50MB directly in your browser.

What are magic bytes?

Magic bytes are a short signature at the start of a file that identifies its container format. A PNG starts with eight fixed bytes, a PDF with “%PDF-”, a ZIP with “PK”. Operating systems use the extension instead, which is why a renamed file can open as the wrong thing or fail completely.

Why does my file show a different type than its extension?

The extension is part of the name and can be edited without touching the content. Common causes are a camera or messaging app renaming a HEIC photo to .jpg, a download saved under the wrong name, or a deliberately disguised executable. The tool reports what the bytes say and names the disagreement.

Does a matching signature mean the file is safe?

No. A signature identifies the container, not the contents. A PDF that begins with “%PDF-” can still contain a malicious script, and a ZIP can contain anything. This tool reads the first 64 KB only; it does not scan for malware.