Attachments are improperly saved when cancelling changes

Reproduction steps:

• Start editing a page and leave the realtime session
• Open the image dialog box and upload an image and insert it
• Cancel the changes

Expected result:

• the uploaded images shouldn't be attached to the edited doc

Obtained result:

• the image is attached to the edited doc even if the changes were cancelled

Note that this behaviour doesn't reproduce if xwiki-realtime is disabled from the administration