# 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 -E -u www-data php occ maintenance:mimetype:update-js

    # you may also need to update the mimetype for existing files, see nextcloud/server#30566
    $ sudo -E -u www-data php occ maintenance:mimetype:update-db --repair-filecache

See [Using the occ command](/link/937) to learn more about `occ`.

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

image  
Generic image

image/vector  
Vector image

audio  
Generic audio file

x-office/document  
Word processed document

x-office/spreadsheet  
Spreadsheet

x-office/presentation  
Presentation

text  
Generic text document

text/code  
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.