doc: instruct users to install mailcap for more specific mime icons

[Equim-chan]

Some machines don't have /etc/mime.types installed by default, making mime.TypeByExtension likely to fallback to blob on most files and therefore will be shown as insert_drive_file in the frontend.

https://github.com/filebrowser/filebrowser/blob/6e1c6a4a8cb1fecfeddf10ada67618c90912336d/file.go#L214

See mime.TypeByExtension.

Related: #376