Allow to edit multiple blobs through WOPI from the same document

Editing an attachment does not work, it always opens the main file, not the selected attachment. The split in WOPIServlet to retrieve the xpath is not correct.

Moreover, seems like when editing the main content of a document through WOPI, it's not possible to edit an attachment as the document is already locked according to Office Online.

It must be possible to edit all blobs from a document at the same time.