FYI, patch no longer works with Drupal 10.3.0. I have not had the time to look into.
Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: You have requested a non-existent service 'mime_type.guesser'
Validation code seems indeed suspicious. I'm getting "Array to string conversion" warnings.
It seems we are sending to validation an array of validators instead of an array of fields?
Don't know much about validation, so I might be totally wrong.
Warning: Array to string conversion in Drupal\file\Plugin\rest\resource\FileUploadResource->validate() (line 45 of /var/www/experience/dev/web/core/modules/rest/src/Plugin/rest/resource/EntityResourceValidationTrait.php)
#0 /var/www/experience/dev/web/core/includes/bootstrap.inc(164): _drupal_error_handler_real()
#1 [internal function]: _drupal_error_handler()
#2 /var/www/experience/dev/web/core/modules/rest/src/Plugin/rest/resource/EntityResourceValidationTrait.php(45): array_diff()
#3 /var/www/experience/dev/web/modules/contrib/webform_rest/src/Plugin/rest/resource/WebformFileUploadResource.php(129): Drupal\file\Plugin\rest\resource\FileUploadResource->validate()
#4 [internal function]: Drupal\webform_rest\Plugin\rest\resource\WebformFileUploadResource->post()
@gdurglishvili Error seems pretty explicit and is not in the module but in the code you use to send the file.
Server is expecting a POST request, and you are doing a GET.
Sorry I had wrong copy/paste in my initial post
The missing one is : "view feedback message entities"
quadbyte → created an issue.