BibDocFile: preferred extension

* When guessing the extension of a file though the
  usage of the magic library and the mimetypes module,
  allow to specify the preferred extension for a
  given guessed mimetype in order to avoid picking up
  a random one from the multiple extensions proposed
  by the mimetype library.
  (closes #1619)

* Introduces CFG_BIBDOCFILE_PREFERRED_MIMETYPES_MAPPING
  new configuration parameter in order to tune the
  above mentioned functionality.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reported-by: Tibor Simko <tibor.simko@cern.ch>
BibDocFile: fix "delete" CLI option
BibUpload: do not always process MoreInfo

* During FFT elaboration, MoreInfo processing is not done if
  BibDoc's have been deleted/expunged/purged. This avoids getting
  an error when using bibdocfile CLI to delete BibDocs.