Add WebP mimetype at startup, not in gallery RSS

Doing it early like this makes the added WebP mimetype available
to the several other places we call things like 'mimetypes.guess_type'.

nikola/nikola.py

@@ -891,6 +891,7 @@ def __init__(self, **config):
                     utils.LOGGER.warning('You are currently disabling "{}", but not the following new taxonomy plugins: {}'.format(old_plugin_name, ', '.join(missing_plugins)))
                     utils.LOGGER.warning('Please also disable these new plugins or remove "{}" from the DISABLED_PLUGINS list.'.format(old_plugin_name))
                     self.config['DISABLED_PLUGINS'].extend(missing_plugins)
+
         # Special-case logic for "render_indexes" to fix #2591
         if 'render_indexes' in self.config['DISABLED_PLUGINS']:
             if 'generate_rss' in self.config['DISABLED_PLUGINS'] or self.config['GENERATE_RSS'] is False:
@@ -1000,6 +1001,9 @@ def __init__(self, **config):
             self.state._set_site(self)
             self.cache._set_site(self)
 
+        # WebP files have no official MIME type yet, but we need to recognize them (Issue #3671)
+        mimetypes.add_type('image/webp', '.webp')
+
     def _filter_duplicate_plugins(self, plugin_list):
         """Find repeated plugins and discard the less local copy."""
         def plugin_position_in_places(plugin):

nikola/plugins/task/galleries.py

@@ -725,9 +725,6 @@ 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']: