diff --git a/sigal/gallery.py b/sigal/gallery.py
index 89ea92a..6d50b2c 100644
--- a/sigal/gallery.py
+++ b/sigal/gallery.py
@@ -132,6 +132,7 @@ class PathsDb(object):
             alb_thumb = self.db[path].setdefault('thumbnail', '')
             if alb_thumb and os.path.isfile(join(self.basepath, path,
                                                  alb_thumb)):
+                self.logger.debug("Thumb for %s : %s", path, alb_thumb)
                 continue
 
             for subdir in self.get_subdirs(path):
@@ -140,6 +141,8 @@ class PathsDb(object):
                     self.db[path]['thumbnail'] = join(
                         os.path.relpath(subdir, path),
                         self.db[subdir]['thumbnail'])
+                    self.logger.debug("Found thumb for %s : %s", path,
+                                      self.db[path]['thumbnail'])
                     break
 
             if not self.db[path].get('thumbnail', ''):
diff --git a/sigal/writer.py b/sigal/writer.py
index 77ad53d..20677be 100644
--- a/sigal/writer.py
+++ b/sigal/writer.py
@@ -176,18 +176,22 @@ class Writer(object):
 
             # generate the thumbnail if it is missing (if
             # settings['make_thumbs'] is False)
-            if not os.path.exists(thumb_path):
+            if not os.path.isfile(thumb_path):
                 source = os.path.join(self.output_dir, dpath, alb_thumb)
                 ext = os.path.splitext(source)[1]
                 self.logger.debug("Generating thumbnail for %s", source)
 
                 if ext in self.settings['img_ext_list']:
                     generator = sigal.image.generate_thumbnail
-                else:
+                elif ext in self.settings['vid_ext_list']:
                     generator = sigal.video.generate_thumbnail
+                else:
+                    generator = None
+                    self.logger.error('Unsupported file type for %s', source)
 
-                generator(source, thumb_path, self.settings['thumb_size'],
-                          fit=self.settings['thumb_fit'])
+                if generator:
+                    generator(source, thumb_path, self.settings['thumb_size'],
+                              fit=self.settings['thumb_fit'])
 
             ctx['albums'].append({
                 'url': d + '/' + self.url_ext,