file upload : check if getvalue attribute exists before calling it

related to https://dev.sourcefabric.org/browse/LBSD-283
The `length` was already added to metadata [here](https://github.com/superdesk/superdesk-server/blob/db8bb73688ba647f0a66cba5fb16e58477aec00e/superdesk/media/media_operations.py#L75) actually. So I remove these lines