Symfony form: Uploaded file - "This value should be of type string"

In config/packages/validator.yaml comment out these lines if they exist:

framework:
    validation:
        # Enables validator auto-mapping support.
        # For instance, basic validation constraints will be inferred from Doctrine's metadata.
        #auto_mapping:
        #  App\Entity\: []

See Symfony 4.3 issue [Validation] Activate auto-mapped validation via an annotation #32070.