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>

koha-tmpl/intranet-tmpl/prog/en/modules/plugins/plugins-upload.tt

@@ -49,7 +49,7 @@
                          <div class="hint"><strong>NOTE:</strong> Only KPZ file format is supported.</div>
                         <ol>
                             <li>
-                                <label for="uploadfile">Select the file to upload: </label><input type="file" id="uploadfile" name="uploadfile" />
+                                <label for="uploadfile">Select the file to upload: </label><input type="file" id="uploadfile" name="uploadfile" accept=".kpz" />
                             </li>
                         </ol>
                     </fieldset>