fix. additional file upload with html5 post method

src/FileUpload.vue

@@ -744,7 +744,11 @@ export default {
       for (let key in file.data) {
         value = file.data[key]
         if (value && typeof value === 'object' && typeof value.toString !== 'function') {
+          if (value instanceof File) {
+            form.append(key, value, value.name)
+          } else {
             form.append(key, JSON.stringify(value))
+          }
         } else if (value !== null && value !== undefined) {
           form.append(key, value)
         }