Cannot get file upload/attachment with Sprout Forms working
Figured out what my problem was: I was trying to submit the form via Ajax, that's why the submit button is set to input type="button" 
I changed the submit button to input type="submit" &amp; it worked.
Not 100% sure how to make it work with Ajax but pretty sure it has something to do with the way the data is serialized. I don't really need Ajax submission in this instance, so not bothering with it anymore.