The .desktop file must include a MimeType key whose value comprises each MIME type to be registered, separated by semicolons. For example: MimeType=text/html;text/css;text/x-javascript
# update-desktop-database
This command creates or updates the mimeinfo.cache file in the same directory as the .desktop file. This cache file is used to facilitate MIME type searches.
The defaults.list file is a plain text file that specifies the default application to use for specific MIME types. Each line in the file consists of the MIME type, the = symbol and the Desktop File ID, which is the filename for the desktop entry file. For more information, see Example 5, A User's defaults.list File.
[Default Applications] application/pdf=evince.desktop text/html=epiphany.desktop text/plain=gedit.desktop image/jpeg=eog.desktop image/png=eog.desktop text/xml=gedit.desktop