MIME Type Lookup

MIME (Multipurpose Internet Mail Extensions) types, also called media types or content types, identify the format of a file or document. They follow the format type/subtype — for example, text/html, application/json, image/png. HTTP uses them in the Content-Type header to tell clients how to interpret the response body. Browsers use MIME types to decide whether to display, download, or run a file.

The HTTP Content-Type header specifies the MIME type of the resource in the response body. For example: Content-Type: application/json tells the client the body is JSON. Content-Type: text/html; charset=utf-8 tells it HTML with UTF-8 encoding. For uploads, the request Content-Type: multipart/form-data specifies a file upload form. Setting the wrong Content-Type causes browser misinterpretation and security issues (MIME sniffing).

MIME sniffing is when browsers ignore the declared Content-Type and infer the actual type from the content itself (e.g., looking for magic bytes). This was historically used to handle misconfigured servers, but it is a security risk — an attacker can serve a .jpg that the browser interprets as HTML and executes scripts. Use the X-Content-Type-Options: nosniff header to disable MIME sniffing and force browsers to respect the declared Content-Type.

application/octet-stream is the MIME type for arbitrary binary data with no specific type. Browsers typically respond to it by downloading the file rather than displaying it. It is commonly used as a fallback when the specific binary type is unknown. For known binary types (PDF, ZIP, executable), use the specific MIME type instead — application/pdf, application/zip, application/vnd.microsoft.portable-executable.

Historically both were used, but text/javascript is now the registered standard for JavaScript (RFC 9239, 2022). application/javascript was briefly the preferred form but is now marked as obsolete. Use text/javascript for the Content-Type of JavaScript responses. For type attributes in script tags, type="module" or no type attribute is preferred — type="text/javascript" is redundant in HTML5.