Bug 18704 - File types limit in tools/export.pl is causing issues with csv files...

Bug 18704 - File types limit in tools/export.pl is causing issues with csv files generated by MS/Excel

To test:
1 - Save a csv of biblionumber from MS/Excel
2 - Attempt to export records using this file
3 - It fails (mimetype is appliction/vnd.ms-excel)
4 - Apply patch
5 - Try again
6 - It succeeds!

Signed-off-by: Jason Palmer <jpalmer@switchinc.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>