Type an extension like .webp or a type like application/json to look it up, or choose a file to see what it really is. The first bytes of a file tell its true format, whatever its name says — handy when a server sends the wrong Content-Type.
Files are read in your browser — only their first bytes are looked at, and nothing is uploaded.
A MIME type — also called a media type or content type — is the label a server puts on a response with the Content-Type header: text/html, image/png, application/json. The browser decides what to do with the response from that label, not from the file name. Send a stylesheet as text/plain and the browser refuses to apply it; send a PDF as application/octet-stream and it downloads instead of opening.
The first part is the family — text, image, audio, video, font, application — the second the exact format. A +json or +xml suffix means "built on JSON or XML".
Text types carry the encoding as a parameter: text/html; charset=utf-8. Leave it out and some browsers guess, which garbles non-ASCII characters.
application/octet-stream means "unknown bytes". It is the safe default for files the server cannot classify, and always triggers a download.
With X-Content-Type-Options: nosniff the browser trusts the header completely. Good for security — but a wrong Content-Type then breaks scripts and styles outright.
Most binary formats start with a fixed signature, sometimes called a magic number: PNG files begin with the bytes 89 50 4E 47, PDFs with %PDF, ZIP files with PK. Reading the first bytes tells you what a file really is, regardless of its name. That is how upload filters catch a script renamed to photo.jpg, and how you find out that a "broken" image is actually an HTML error page saved with an image extension.
Office documents (docx, xlsx, pptx), EPUB and Android packages are all ZIP archives inside; the checker looks into the first entries to tell them apart. Text formats like JSON, SVG, CSV and HTML have no signature, so they are recognised by their opening characters.
When an API or asset suddenly breaks in the browser, check the Content-Type the server sends as well — an HTTP monitor can alert you when it changes.
application/json. Since 2006 it has been the only registered type for JSON; text/json and text/javascript are outdated and should not be used for API responses.
text/javascript. The HTML standard settled on it; application/javascript still works in every browser but is listed as obsolete. Modules (.mjs) use the same type.
The server sends it as application/octet-stream or another type the browser cannot display, or adds Content-Disposition: attachment. Configure the server to send the correct type for that extension.
Yes — the extension is just part of the name. The signature bytes at the start of the file show the real format, which is why upload validation should check the content and not only the name.
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