Add drop target to main blob previewer

This request corresponds to the backport of NXP-24084 to LTS 2017

Problem
When changing main file, users need to delete and DnD new file.
Before, this was also due to users not losing main files on a mistake.
Today, with version policies, it can be setup version so this is not an issue.

Proposed solution
Add the preview as a drop target. When a file is dropped, it replaces the main file.

Decided implementation

Display dropzone below main file info (only available when there are write permissions).
Display document thumbnail in the preview area when there isn't main file yet (which in this case will fallback to the doc type image).

Note
This may bring up mimetype issues for some doctypes (ie. picture), but that is currently already the case.