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>