The Files and IIIF imports should not push the DataFiles as artifacts

1. Upload an image, a PDF, a IIIF manifest or a Transkribus export on the frontend.
2. Start a file import.
3. Wait for the file import to complete.
4. Click on the Artifacts button.
5. The file you uploaded is listed in there.
6. Cry because you're wasting disk space.

ProcessTask.download_files() should download DataFiles into a temporary directory instead.