Bug 33189: Make plugin upload look for .kpz files in file picker

Bug 33189: Make plugin upload look for .kpz files in file picker

This change configures the file picker to look for *.kpz files instead
of *.* files, which makes it more convenient to locate .kpz files
for upload when uploading plugins.

Test plan:
0. Apply patch
1. Go to http://localhost:8081/cgi-bin/koha/plugins/plugins-upload.pl
2. Click on "Choose file"
3. Note that the file type in the box at the bottom right of the screen
defaults to "KPZ File (*.kpz)
4. If you navigate to a directory that contains .kpz files (like Downloads),
you'll now see directory folders and .kpz files.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>