Fix ActiveModel::Errors#[]= deprecation warning

DEPRECATION WARNING: ActiveModel::Errors#[]= is deprecated and will be
removed in Rails 5.1. Use model.errors.add(:attachment, "content type
image/png does not match any of accepted content types pdf") instead.
(called from validate_attachment_content_type at
/home/travis/build/consul/consul/app/models/document.rb:92)

app/models/direct_upload.rb

@@ -51,7 +51,7 @@ class DirectUpload
     @relation.valid?
 
     if @relation.errors.key? :attachment
-      errors[:attachment] = @relation.errors[:attachment]
+      errors.add(:attachment, @relation.errors.full_messages_for(:attachment))
     end
   end