diff --git a/sphinx/transforms/post_transforms/images.py b/sphinx/transforms/post_transforms/images.py
index e4f1c29cb..ab7b75837 100644
--- a/sphinx/transforms/post_transforms/images.py
+++ b/sphinx/transforms/post_transforms/images.py
@@ -16,6 +16,7 @@ from docutils import nodes
 
 from sphinx.transforms import SphinxTransform
 from sphinx.util import logging, requests
+from sphinx.util.images import guess_mimetype
 from sphinx.util.osutil import ensuredir
 
 if False:
@@ -74,8 +75,9 @@ class ImageDownloader(BaseImageConverter):
                 with open(path, 'wb') as f:
                     f.write(r.content)
 
+                mimetype = guess_mimetype(path, default='*')
                 node['candidates'].pop('?')
-                node['candidates']['*'] = path
+                node['candidates'][mimetype] = path
                 node['uri'] = path
                 self.app.env.images.add_file(self.env.docname, path)
         except Exception as exc:
diff --git a/sphinx/util/images.py b/sphinx/util/images.py
index 8de8254db..7fc89f8aa 100644
--- a/sphinx/util/images.py
+++ b/sphinx/util/images.py
@@ -52,8 +52,8 @@ def get_image_size(filename):
         return None
 
 
-def guess_mimetype(filename):
-    # type: (unicode) -> unicode
+def guess_mimetype(filename, default=None):
+    # type: (unicode, unicode) -> unicode
     _, ext = path.splitext(filename)
     if ext in mime_suffixes:
         return mime_suffixes[ext]
@@ -63,4 +63,4 @@ def guess_mimetype(filename):
             if imgtype:
                 return 'image/' + imgtype
 
-    return None
+    return default