FIX: Add onceoff job to fix incorrect extension for gravatar uploads.

app/jobs/onceoff/fix_invalid_gravatar_uploads.rb

@@ -0,0 +1,14 @@
+module Jobs
+  class FixInvalidGravatarUploads < Jobs::Onceoff
+    def execute_onceoff
+      Upload.where(original_filename: "gravatar.png").find_each do |upload|
+        extension = FastImage.type(Discourse.store.path_for(upload))
+        current_extension = upload.extension
+
+        if extension.to_s.downcase != current_extension.to_s.downcase
+          upload.user.user_avatar.update_gravatar!
+        end
+      end
+    end
+  end
+end