Improvement of the file attachment process

[ArtificialOwl]

Some possible look&feel improvements during the file attachment process:

• ~~[ ] Only show the 'close' badge when hovering on an image~~
• [ ] Nicer generic icon for non image file types.
• [ ] Back-end should support non image files (@daita)
• [ ] Support attaching several files at once.

[miklobit]

• [ ] Support for attaching media selected from the "Files" module. (currently only local files can be selected )