Fixed bug with json files content being escaped in api

After update to recent grape env['api.format'] does not work any more.
Use content_type for rendering raw json files content
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

lib/api/api.rb

@@ -22,6 +22,8 @@ module API
     end
 
     format :json
+    content_type :txt, "text/plain"
+
     helpers APIHelpers
 
     mount Groups

lib/api/repositories.rb

@@ -167,9 +167,7 @@ module API
         blob = Gitlab::Git::Blob.find(repo, commit.id, params[:filepath])
         not_found! "File" unless blob
 
-        env['api.format'] = :txt
-
-        content_type blob.mime_type
+        content_type 'text/plain'
         present blob.data
       end