Replace value 2 for the fileStage during submission file creation by a PKP constant

[tnier01]

The SUBMISSION_FILE_SUBMISSION is a PHP constant that is equal to 2. So that’s why it’s working when you use the integer. However, you should use the constant to protect yourself in case the value changes.

If you’re on the main branch, you can use the following:
'fileStage' => PKP\submission\SubmissionFile::SUBMISSION_FILE_SUBMISSION instead of
'fileStage' => 2,