Check whether an extension is suspicious
When a downloaded file's extension does not match its content, use the header signature to confirm what it really is.
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Guide
File Info reads the file metadata the browser provides and guesses the real format from a few bytes at the start of the file using a built-in signature table. If the MIME type is an image, a local Object URL loads a preview and the natural width and height are read as well; SHA-256 gives the content digest.
Updated 2026-09-272 min read
If a file named .txt starts with the PNG signature, the page shows “browser MIME / extension” and “file header detection” as two separate results, rather than treating the extension as the only truth.
SNIFF_BYTES, and the signature table has limited coverage; “unrecognised” can still be ordinary text or a rare format.When a downloaded file's extension does not match its content, use the header signature to confirm what it really is.
Take the name, size, MIME type and modified time and paste them into a ticket or a record.
Verify that downloaded content matches the digest the publisher announced.
No. It only matches some format signatures; it does not scan for malicious code.
File contents and the preview Object URL stay in the browser and are never uploaded; the preview URL is released when the page unloads.
Updated 2026-09-27
Name, size, MIME, modified time, file header, SHA-256 and image dimensions at a glance
Metadata is read locally: name / size / MIME / file header / SHA-256 / image size