Add logic to detect (from added URLs and uploaded files) resources' content/mime/file type
Needs to check the content of the file (and not just the extension or mime type):
- when uploading a file (which might imply further modifying our fork of furlex, or switching to something better)
- at the same time as extracting the metadata of a URL
We can discuss whether to store this as one or both of:
- mime type
- file extension