Fix thumbnail generation when picture is uploaded through WebDAV

Thumbnail is not generated because WebDAV (at least Windows client) does not provide a content type, so it is set to "application/octet-stream" which prevents from computing metadata

Quick fix: if mimetype is "application/octet-stream", we should use imaging service to get the real mime type (as it's done when mime type is null)

Other fix: do not set mime type to "application/octet-stream" in WebDAV code