How to use
In “Search” enter an extension (xlsx), a MIME (application/json) or a word (video). Candidates appear labelled official, common or deprecated.
Drop a file into “Check a file” to compare the MIME implied by its extension, the file.type the browser assigned, and the type detected from its first bytes. Mismatches are flagged.
Paste a response header value into “Content-Type” to split it into type, subtype, +suffix and parameters and see grammar problems. You can also build a value with a charset below.
In “Make config” list extensions and pick nginx, Apache, .htaccess, Express or S3 for a draft. The download Content-Disposition builder is here too.
Read “Field notes” for pitfalls with JSON, JavaScript, SVG, WebAssembly, fonts, octet-stream and Office’s long MIME types.
What it does
Official, common and deprecated kept apart
IANA-registered values, de-facto values that servers and operating systems really use, and deprecated ones such as application/javascript or image/jpg are labelled separately, with the replacement shown. When an extension has several candidates they are ordered by preference.
Checked against the real content
The extension, the browser’s file.type and the first-bytes signature are shown side by side, catching executables or text posing as images. .docx, .xlsx and .pptx are checked down to the ZIP structure to tell them from a renamed plain ZIP. Files are never uploaded.
Content-Type parsed with RFC 9110 grammar
Tokens, quoted strings, escapes and whitespace are handled as the RFC specifies. It flags charset placement, spaces around “=”, duplicate parameters, unknown charsets, missing boundaries and merged multiple values, and prints a cleaned value.
Config drafts and download headers
Generates an nginx types block, Apache AddType, .htaccess, Express express.static and S3 aws commands plus SDK code. Non-ASCII file names get a Content-Disposition with filename* alongside an ASCII fallback.
Use cases
- Check that an uploaded file’s extension matches its content
- Write server config for a new extension such as .wasm, .webp or .woff2
- Trace a “Refused to execute script … MIME type” error to its Content-Type
- Copy the long Excel or Word MIME type exactly
- Build a Content-Disposition for downloads with non-ASCII file names
Supported formats
Input
Capability matrix
| Feature | Support | Notes |
|---|---|---|
| Extension ⇔ MIME search (partial match, descriptions) | Supported | — |
| Extension vs file.type vs content mismatch check | Supported | — |
| Content-Type parsing and diagnostics (RFC 9110 grammar) | Supported | — |
| nginx / Apache / .htaccess / Express / S3 config generation | Supported | — |
| Content-Disposition for non-ASCII file names | Supported | — |
| Every MIME type in existence | Partial | A table of the main formats, not the full IANA registry |
| Detection from file content | Partial | Covers the main image, audio, video, document, archive and font formats; text types cannot be confirmed |
| Fetch a server’s real response headers | Not supported | No network access. Run curl -I or use devtools and paste the value |
| Virus or malware scanning | Not supported | It reports format mismatches, not safety |
Limits & notes
- This tool makes no requests. Read the response headers with curl -I or browser devtools and paste the value.
- The table covers the main formats and is not exhaustive. A MIME that is not listed can still be checked for grammar.
- File checks use a 4 KB signature. For ZIP-based files only the central directory at the end is read to list entry names (ZIP64 and damaged ZIPs fall back to the extension). At most 50 files at a time.
- The browser’s file.type depends on OS registration, so it differs between machines.
- Default tables of nginx and Apache vary by version. Validate generated config with each server’s syntax check before applying it.
Privacy & safety
Text you enter and files you drop are processed in your browser only. Nothing is sent to or stored on a server.
FAQ
What is the MIME type of .xlsx?
Is JavaScript not application/javascript?
Can I use image/jpg?
What does X-Content-Type-Options: nosniff do?
Is a chosen file uploaded?
How do I serve a download with a Japanese file name?
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