MIME type
A MIME type (media type) is a standardized label like image/png or application/json that tells clients what kind of data a file contains. Sent in the Content-Type header, it tells browsers whether to render, download, or parse a response, and lets servers validate uploads by format.
A MIME type has a type and subtype separated by a slash — image/png, text/html, application/pdf, video/mp4. The browser uses it to decide behavior: image/png renders inline, application/octet-stream triggers a download, text/html is parsed as a document. Get it wrong and a PDF might download instead of displaying, or a script might be served as plain text.
On the storage side, the Content-Type you set when uploading is stored with the object and returned on every fetch. If you don't set it, many systems fall back to application/octet-stream, which forces downloads. Always set the correct type at upload time.
MIME types are also a security boundary. Validating the declared type — and not blindly trusting a file extension — is part of safely accepting user uploads, since a .jpg can contain anything.