Bug 18087: Handle invalid filetypes

Bug 18087: Handle invalid filetypes

If an invalid file is used as biblionumber list, we should display a
message.

Test plan:
1/ Use csv, plain text files
=> Should work
2/ Use invalid files (binary files like pdf, doc*, xsl*, etc.)
=> Should not work and see a warning message.

Amended patch after signoff: Remove one warn debug line

Signed-off-by: Joy Nelson <joy@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit d43131c4fca8124a8a9e4dcdea82e2bdf1af196a)
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>