Allow CMIS clients to create images as Picture objects

Some AtomPub CMIS clients (Adobe Drive for instance) create an image with the following sequence:

• create doc in checked out state with atom:title="foo.jpg" and CMIS type "cmis:document" (POST in children collection)
• set the blob object on it (PUT on content)
• check in (PUT on entry with checkin=true parameter)

The problem is that this creates a File document, instead of a Picture.

It would be better to use Picture instead of File if we detect that it's going to be an image. The only information we have to decide that is the atom:title.