Unit Testing File Upload Postprocessing

Hello everyone!

I’ve been trying my hands on a somewhat stateless file upload: User uploads a file, the controller processes it and returns a form with pre-filled out fields based on the values from the file. The uploaded file is not saved in any manner.
Fortunately, I was successful :stuck_out_tongue:
Now, I want to write unit tests for this function, and I am stuck as to how I would approach that.
I’ve tried already to use setFiles() in the test case like so:

$request = $this->getRequest();
$request->setFiles(new Parameters(
                    'fileUpload' => [
                        'name' => 'project.xml',
                        'type' => 'text/xml',
                        'tmp_name' => __DIR__.'/project.xml',
                        'error' => 0,
                        'size' => 128885
        $this->dispatch('/thing/add', 'POST', ['importButton' => 'Import']);

But PHP considers this ‘an illegal file upload’ (which is understandable imho).
How would I ‘legally’ upload a file in this scenario? Or is another approach such as overwriting the $form->isValid() (in the controller) via stub more viable?

Thanks a lot!