How to test the checksum of an uploaded file

I can't see any specific issues with this - if it were me I would check that the file was created in the /config directory and had the correct permissions etc. It could be the case that it takes a short amount of time between upload and checking ( because upload is normally to a temp directory and the file is then moved once it is complete ) so including a 1 second sleep might be a good idea as well. Have you tested the method using a known-present external datagroup file?