Site metafields throw no_method_error on type: file fields on engine

I have something like this:

slider:
  fields:
    slider_text:
      type: string
    slider_bild_1
      type: file

(metafields_schema.yml)

On wagon, this works perfectly.
On the engine, I get this (self-hosted and on locomotivehosting):

I, [2016-07-16T17:51:02.759050 #3087]  INFO -- : [LocomotiveCMS] [ApplicationError] #<NoMethodError: undefined method `slider_bild_1?' for #<Locomotive::CurrentSiteMetafieldsHelper::SchemaGroup:0x00000005
6cbe20>>
I, [2016-07-16T17:51:02.762500 #3087]  INFO -- : Completed 500 Internal Server Error in 87ms
F, [2016-07-16T17:51:02.772287 #3087] FATAL -- :
NoMethodError (undefined method `slider_bild_1?' for #<Locomotive::CurrentSiteMetafieldsHelper::SchemaGroup:0x000000056cbe20>):

(log excerpt from self-hosted engine)

=> The file field causes an error (the text field alone works).

Since it's an image field, I changed it to type: image, and this seems to solve the issue.

However, I can't get type: file to work.