Fix missing Drop-and-Replace file feature in WebUI

In WebUI, in a workspace, create a file document and associate a file as a primary content.
To replace this primary content, you must first suppress it then the drag-n-drop possibility reappears and enables you to drop a new primary content version in place.

Customers want a way to replace a primary content via drag-n-drop without having first to suppress the primary content.

Customers expect the same functionality with WebUI than with JSF: for a File document, for instance, being able using drap-n-drop to trash-and-replace an existing primary file with a new instance of the file.