validate blank images

app/controllers/api/attachments_controller.rb

@@ -10,6 +10,12 @@ module Api
     def create
       submitter = Submitter.find_by!(slug: params[:submitter_slug])
 
+      if params[:type].in?(%w[initials signature]) && ImageUtils.blank?(Vips::Image.new_from_file(params[:file].path))
+        Rollbar.error("Empty signature: #{submitter.id}") if defined?(Rollbar)
+
+        return render json: { error: "#{params[:type]} is empty" }, status: :unprocessable_entity
+      end
+
       attachment = Submitters.create_attachment!(submitter, params)
 
       if params[:remember_signature] == 'true' && submitter.email.present?