Fix exception from gallery RSS with WebP images

WebP images in a gallery cause `nikola build` to fail with an unhandled
AttributeError exception, as described in getnikola#3671.

This happens because the MIME type of WebP images is not known, and the
resulting None value causes the RSS generation code to choke.

This PR fixes that, by augmenting Python's list of known mimetypes with
a hardcoded WebP one, before we ask for the mimetype of each image in a
gallery.

This addresses the above issue's checklist item "*Inject a hardcoded
'image/web' mimetype at runtime, before .guess_type() gets called.*",
but does not complete the issue.

nikola/data/samplesite/galleries/demo/metadata.sample.yml

@@ -7,7 +7,7 @@ order: 2
 name: tesla4_lg.jpg
 order: 0
 ---
-name: tesla_conducts_lg.jpg
+name: tesla_conducts_lg.webp
 caption: Nikola Tesla conducts electricity
 order: 1
 ---

nikola/plugins/task/galleries.py

@@ -725,6 +725,9 @@ def gallery_rss(self, img_list, dest_img_list, img_titles, lang, permalink, outp
         def make_url(url):
             return urljoin(self.site.config['BASE_URL'], url.lstrip('/'))
 
+        # WebP is not yet an official MIME type but we need to recognize them
+        mimetypes.add_type('image/webp', '.webp')
+
         all_data = list(zip(img_list, dest_img_list, img_titles))
 
         if self.kw['sort_by_date']: