Bug 14647: Make the file name extension dynamic on exporting records

Bug 14647: Make the file name extension dynamic on exporting records

Test -
1. go to cgi-bin/koha/tools/export.pl
2. Check that both tabs ("Export bibliographic records & Rxport authority records") do the following
3. In Output format
4. When changing "File format" from dropdown it update "file name"
5. Still works with user inputted names

Works as expected.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>