Rework the File Upload feature - make clearer

[markuso]

The file upload feature now might be a bit confusing where when you upload a file you must save the record before you can actually use it inside the content. Plus it is not clear that you can double-click on an image to remove it from the attachements of the document, and that with a single-click on an image you are designating that image as the primary one.

Maybe just documenting those facts somewhere around the upload feature is more than enough, but it can surely be clearer.