Support for icons for mime types is missing

Problem/Motivation

Based on #2878378-12: Visual look of multi file field is not good and latter discussion in this issue.

We should provide this icons and needed CSS - either we reuse current core icons or we introduce font or svg icons with FontAwesome or some other system.

Advantage of having FontAwesome is that we have more icons later for other stuff in child themes.

If we are going to solve this with FontAwesome then this must not be hard coded solution - child themes should be able to easily override icon system and provide another one.

Proposed resolution

Not sure for now.

Remaining tasks

Investigate the best way to do this.

User interface changes

API changes

Data model changes