Add before event to upload files mixin

upload-files.js

@@ -36,6 +36,8 @@ export const upload = function(superClass) {
           formData.append(files[i].name, files[i]);
         }
 
+        document.dispatchEvent(new CustomEvent('before-request', { detail: request, bubbles: true, composed: true }));
+
         request.send(formData);
       });
     }