file_upload.js.coffee

$form.fileupload
  dataType: "json"
  done: (e, data) ->
    result = if data.result?
      # XHR (as used by, say, Chrome) gives you a simple JSON object
      data.result
    else
      # iframe (as used by IE) gives you text back that you'll want to parse manually)
      # Urgh. UTF8 value (passed by Rails form) causes JSON to fall over. Crappy solution on next line.
      data = JSON.stringify(data).replace(/"name":"utf8","value":"[^"]*"/, '"name":"utf8","value":"REMOVED"')
      JSON.parse(data).result
    console.log result

my_controller.rb

class MyController < ApplicationController

  def my_action
    # Do whatever you need to do
    respond_to do |format|
      format.js { 
        render json: to_json(@agent) 
      }
      format.html { 
        # iframe won't use .js it will just appear as .html - fool the client into thinking they're getting proper json back.
        render text: "<textarea data-type=\"application/json\">#{to_json(@agent)}</textarea>" 
      }
    end
  end
  
end

Some tweaks I had to make to get jQuery-File-Upload working with IE (without breaking other browsers in the process). See https://github.com/blueimp/jQuery-File-Upload