Merge the Images and PDF imports into a File import

DataImportMode.PDF and DataImportMode.Images are merged into a DataImportMode.Files.

The decision to call the PDF or image extraction functions in the import task should be then made based on the MIME type instead of the mode (application/pdf → PDF, image/* → image, anything else → die in mysterious circumstances).

Please add a unit test that includes both an image and a PDF to verify that we can support both at once.