Install jquery-file-upload assets for rails.

app/controllers/documents_controller.rb

@@ -3,7 +3,13 @@ class DocumentsController < ApplicationController
   before_filter :find_documentable, except: :destroy
   before_filter :prepare_new_document, only: :new
   before_filter :prepare_document_for_creation, only: :create
-  load_and_authorize_resource
+
+  before_filter :validate_upload, only: :upload
+  load_and_authorize_resource :except => [:upload]
+  skip_authorization_check :only => [:upload]
+
+  def preload
+  end
 
   def new
   end
@@ -27,6 +33,19 @@ class DocumentsController < ApplicationController
     redirect_to params[:from]
   end
 
+  def upload
+    document = Document.new(documentable: @documentable, attachment: params[:file].tempfile, title: "faketitle", user: current_user )
+    debugger
+    # We only are intested in attachment validators. We set some fake data to get attachment errors only
+    if document.valid?
+      # TODO: Store file on tmp cache
+      msg = { status: 200, msg: "OK" }
+    else
+      msg = { status: 422, msg: document.errors[:attachment].join(', ') }
+    end
+    render :json => msg
+  end
+
   private
 
   def find_documentable
@@ -47,4 +66,8 @@ class DocumentsController < ApplicationController
     params.require(:document).permit(:title, :documentable_type, :documentable_id, :attachment)
   end
 
+  def validate_upload
+    if @documentable.present?
+    end
+  end
 end