BibDocFile: support new magic library

• There are two flavours of the magic library (for file format guessing): the original bindings coming with the CLI file utilities, and the new python-magic bindings wrapping libmagic using ctypes. Support for the new library is here introduced (while still allowing for the old one). (closes #1207)

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>