Move the global file_fields variable at the top of workbench_utils.py to a configuration setting

[mjordan]

No need for it to be a global variable. Also, putting it in a config setting will allow users who have custom media types to add their own file field names.

[mjordan]

Welp, turns out we already have a config setting media_type_file_fields, which has a default of:

            {
                "file": "field_media_file",
                "document": "field_media_document",
                "image": "field_media_image",
                "audio": "field_media_audio_file",
                "video": "field_media_video_file",
                "extracted_text": "field_media_file",
                "fits_technical_metadata": "field_media_file",
                "remote_video": "field_media_oembed_video",
            }

This config setting is only changed if using a custom media type that isn't in the list above. Therefore, we can replace file_fields with a list of values from the above default setting and document how to use it in various tasks.