Bug 1338257 - Remove remaining legacy generator from testing/marionette/. r=ato

testing/marionette/interaction.js

@@ -231,7 +231,7 @@ interaction.selectOption = function (el) {
  * @param {string} path
  *     Full path to file.
  */
-interaction.uploadFile = function (el, path) {
+interaction.uploadFile = function* (el, path) {
   let file = yield File.createFromFileName(path).then(file => {
     return file;
   }, () => {