MIME types

20 October 2021

A MIME type identifies the type of a file. When a server sends resources to a web browser it includes the MIME type of each file, and web browsers use that information to determine how to handle files. For instance, when a browser gets a file with the MIME type text/html it knows it needs to render it as HTML. Or, when it sees the image/jpeg type it knows the file is a JPG (or JPEG) image. For some types it might need to perform some specific magic. PDFs (application/pdf), for instance, are typically opened in the browser’s PDF viewer.

The media types standard is managed by IANA, and you can find a full list of MIME types on their website.

Types, subtypes and parameters

MIME types have a type, subtype and optional parameter:

• The type is a broad category. For instance, there are types such as application, audio, video and font.
• Each type has a large number of subtypes. The audio type, for instance, contains subtypes such as aac, mp4, ogg and opus (there are dozens more).
• A parameter provides additional information. A common example is “charset=UTF-8”, which specifies that the (plain text) file uses UTF-8 character encoding.

The file utility

On Linux servers you can check the MIME type using the file utility with the --mime-type option. Here are some examples:

$ file --mime-type index.html
index.html: text/html

$ file --mime-type index.php
index.php: text/x-php

$ file --mime-type robots.txt
robots.txt: text/plain

$ file --mime-type logo.jpg
logo.jpg: image/jpeg

$ file --mime-type backup.zip
backup.zip: application/zip

The file utility looks at the file itself to determine the MIME type. For binary files, such as images, it checks the signature (also called a magic number). For instance, PNG files use the hexadecimal value 89 50 4E 47 0D 0A 1A 0A as its signature. You can view the value with the xxd utility:

$ xxd image.png | head -1
00000000: 8950 4e47 0d0a 1a0a 0000 000d 4948 4452  .PNG........IHDR

$ file --mime-type image.png
image.png: image/png

Plain text files don’t have a magic number. To check the file type it inspects the file itself.

One thing to be aware of is that file extensions have no meaning on Unix-like servers. File extensions exist purely for your convenience. It is perfectly fine to have, say, a PHP script without an extension – it doesn’t change the file’s MIME type. And you could give a PHP script and extension such as .txt or .png as well. In fact, this is sometimes the case when a website has been compromised – a malicious PHP script may be hiding as an innocent looking plain text or image file.

MIME types on cPanel servers

On cPanel servers you can see all existing MIME types via Advanced » MIME Types. You can’t change existing MIME types via the interface (they are managed at the server level). You do have the option to add a new MIME type. This can be useful if a new MIME type emerges that the server doesn’t know about yet. In practice, it is extremely unlikely this will happen. If you suspect a MIME type is missing, please contact us and we will add it if needed.