If for any case, ajax file upload solution is not working for you, you can switch to non-ajax solution.
To choose non-ajax file upload, follow those steps:
1. Go to User Meta >> Fields Editor, expand your file upload field (or avatar field) and check “Disable AJAX upload” checkbox, save your change.
2. Go to User Meta >> Forms Editor, expand your form and check “Do not use AJAX submit” checkbox and save your change.