I have the same issue:
Couldn't upload file: oo.jpg.
Symfony\Component\Routing\Exception\MethodNotAllowedException: in Drupal\Core\Routing\MethodFilter->filter() (line 46 of /var/www/html/web/core/lib/Drupal/Core/Routing/MethodFilter.php).
The werid thing is that it is working after I go to my user profile and change "Administration pages language" to "No preference".
It's not only depracation error. It's impossible to submit a form with file upload field added.