How to Find MIME Types

Problem

You need to set the correct Content-Type header for a file or determine what type a file is from its extension.

Prerequisites

• Understanding of HTTP headers
• Knowledge of file types

Steps

1. 1. Look up by file extension

   Map the file extension to its MIME type: .json → application/json, .html → text/html, .png → image/png, .pdf → application/pdf. Use a comprehensive MIME type database for accurate mapping, as some extensions have multiple registered types.

   Input

   .json

   Output

   application/json

2. 2. Handle ambiguous extensions

   Some extensions map to multiple MIME types depending on context: .xml can be text/xml or application/xml, .js can be text/javascript or application/javascript. Use the most widely compatible type. For web serving, follow the IANA registry.

   Input

   .xml

   Output

   application/xml (preferred)

3. 3. Set the Content-Type correctly

   When serving files, set the Content-Type header to the correct MIME type. Browsers use this to determine how to handle the response. An incorrect type can cause a file to download instead of display, or render incorrectly. Add charset for text types: text/html; charset=utf-8.

   Input

   serving a JSON file

   Output

   Content-Type: application/json; charset=utf-8

Expected result

The correct MIME type for a file extension, or the correct extension for a MIME type.

Common errors

Wrong Content-Type causes file to download

Set the correct MIME type. For example, text/html for HTML, application/json for JSON. A wrong type like application/octet-stream causes download.

Missing charset for text types

Add charset=utf-8 to text MIME types (text/html, text/plain, application/json). Without it, the browser may guess the encoding incorrectly.

Troubleshooting

• If a file renders as plain text, the Content-Type is likely text/plain instead of the correct type.
• For custom file types, use application/octet-stream as a fallback.

Security limits

• MIME type sniffing can be a security risk. Set X-Content-Type-Options: nosniff to prevent browsers from guessing the type.