MIME Type Lookup

Almost always a wrong or missing Content-Type header. Serve a PDF as application/octet-stream and the browser downloads it rather than opening the viewer; serve it as application/pdf and it displays. The file itself is fine — the header is what tells the browser how to treat it.

No. The Content-Type on an uploaded file comes from the client and is trivially forged, so validating it alone is a well-known security hole — an executable can claim to be an image. Check the actual file contents server-side rather than believing the declared type.

History. JPEG appears as both image/jpeg and the older image/jpg, and JavaScript has been text/javascript, application/javascript and application/x-javascript at various points. Use the current registered type — the alternatives usually still work, but standardising avoids surprises in stricter environments.

Yes, MIME Type Lookup is completely free. No signup, no account, and no watermark on outputs. A Pro tier is available for 100 AI ops/day and larger file sizes.

MIME Type Lookup runs entirely in your browser. Your file is never uploaded — it is read, processed and saved locally, so it never reaches our servers or anyone else's. You can disconnect from the internet after the page loads and it still works.