src/Setup/Linux: Specify icon in mime info

[joebonrichie]

Otherwise file managers will just use a generic missing icon icon.

[joebonrichie]

Nautilus for example will not an icon without this patch

[Jertzukka]

This is what they currently look on my system:

I think this would have to be <generic-icon name="veracrypt"/>, or at least on my GNOME system it won't work otherwise. But even so, I think having the application icon being the icon for a container might be confusing. There probably exists some kind of generic icon that represents an encrypted file that could be used if it were to be changed.