Improve visibility of images uploading process in Image dialog UI

Discussed with Thiago Krieck , a possible solution would be to reuse the attachment upload used in the document footer Attachment tab.

Note that this could also be reused in the user profile picture uploader to improve consistency further.

This attachment uploader does not have a "upload" confirmation button, which would be a significant feature change that we probably want to discuss on the forum. AFAIK there's no specific reason for this upload confirmation to be here.