posts: fix posts with .jpeg file extension.

There used to be about 1000 posts with a .jpeg file extension instead of
.jpg. These posts have been fixed manually, so we no longer have to
check for this any more.

app/models/post.rb

@@ -182,7 +182,7 @@ class Post < ApplicationRecord
     end
 
     def is_image?
-      file_ext =~ /jpg|jpeg|gif|png/i
+      file_ext =~ /jpg|gif|png/i
     end
 
     def is_png?