Improve file attachement usability

UX of file attachements is quite bad if there's more than two or three files attached. We oftenly use crabgrass pages as dropbox-style collaboration tool (e.g. for flyers, posters, different versions etc).

Ideas for better usability:

• Allow to switch from the thumbnail preview to a list mode. Thumbnails are quite unusable for everything but images.
• Allow to expand the file attachement from the sidebar to the main content field
• Allow to sort the file attachements by name or by date added/updated
• Include dropzone.js or similar directly in the frontend to add/update attachements