Can't register .xls as custom MIME type in Rails4

I updated config / initializers / mime_types.rb and restarted the server.

Mime::Type.register "application/vnd.sealed-xls", :xls

      

        
        
        
      

    

My controller looks like this:

respond_to do |format|
  format.xls {  send_file(file_name, filename:  "dagsrapport.xls")  }
  format.html
end

      

        
        
        
      

    

This is the error I get when I do a GET on /sheet.xls

To respond to a custom format, register it as a MIME type first: 
http://guides.rubyonrails.org/action_controller_overview.html#restful-downloads. If you meant to 
respond to a variant like :tablet or :phone, not a custom format, be sure to nest your variant 
response within a format response: format.html { |html| html.tablet { ... } }

      

        
        
        
      

    

Any tips on what I'm missing? I can't see what I am doing wrong according to the doc: http://guides.rubyonrails.org/action_controller_overview.html#restful-downloads