determine filename for upload-file and remote-iri

.original_filename only works for UploadFiles not for any other Tempfile
(of course, as Tempfiles do not have an origin). So we'll need to refer
to the iri instead. We'll just take the part after the last slash '/' as
the filename.
add (currently synchronous) remote file-retrieval

We should make this job asynchronous later on. However for now
synchronous is good enough.