JSON request, show error message

Hello I am trying to show the errors that can occur by uploading a file (file name, size). I tried to modify this code cut-off but I could not.

else    format.html { render action: "new" }    format.json { render json: @upload.errors, status: :unprocessable_entity } end

I have following validations and I want json to write when an error occured, so something like this:

else    format.html { render action: "new" }    format.json { render json: @upload.errors, status: :unprocessable_entity, error:"Error!" } end

(it doenst work) and I need a specification of an error, that is written in a validation :message

my validations:

validates_uniqueness_of :upload_file_name, :message => "File with this name is already in the database"

validates :upload_file_size, :inclusion => {:in =>10.megabytes..20.megabytes}, :message =>"Too big or too small"

      def to_jq_upload         {           "name" => read_attribute(:upload_file_name),           "size" => read_attribute(:upload_file_size),           "url" => upload.url(:original),           "delete_url" => upload_path(self),           "delete_type" => "DELETE"         }       end

    end

Thanks in advanced