Bug 18087 - Clarification on File type when using file of biblionumbers to export...

Bug 18087 - Clarification on File type when using file of biblionumbers to export data

Added a line to the screen  detailing the types of files that can be used to upload a list of biblionumbers.

Test plan:
1. Go to tools->export data
2. under File option,  (File types accepted: .csv and .txt)  should appear
3. Check both Bibliographic and Authority Export screens for this new string.

Signed-off-by: Claire Gravely <claire_gravely@hotmail.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mason James <mtj@kohaaloha.com>