Mimetypes management

Mimetype aliases

Nextcloud allows you to create aliases for mimetypes, so that you can display custom icons for files. For example, you might want a nice audio icon for audio files instead of the default file icon.

By default Nextcloud is distributed with nextcloud/resources/config/mimetypealiases.dist.json. Do not modify this file, as it will be replaced when Nextcloud is updated. Instead, create your own nextcloud/config/mimetypealiases.json file with your custom aliases. Use the same syntax as in nextcloud/resources/config/mimetypealiases.dist.json.

Once you have made changes to your mimetypealiases.json, use the occ command to propagate the changes through the system. This example is for Ubuntu Linux:

$ sudo -u www-data php occ maintenance:mimetype:update-js

See Using the occ command to learn more about occ.

Some common mimetypes that may be useful in creating aliases are:


Generic image


Vector image


Generic audio file


Word processed document






Generic text document


Source code

Mimetype mapping

Nextcloud allows administrators to specify the mapping of a file extension to a mimetype. For example files ending in mp3 map to audio/mpeg. Which then in turn allows Nextcloud to show the audio icon.

By default Nextcloud comes with mimetypemapping.dist.json. This is a simple json array. Administrators should not update this file as it will get replaced on upgrades of Nextcloud. Instead the file mimetypemapping.json should be created and modified, this file has precedence over the shipped file.

Icon retrieval

When an icon is retrieved for a mimetype, if the full mimetype cannot be found, the search will fallback to looking for the part before the slash. Given a file with the mimetype ‘image/my-custom-image’, if no icon exists for the full mimetype, the icon for ‘image’ will be used instead. This allows specialised mimetypes to fallback to generic icons when the relevant icons are unavailable.