That's weird :/ I haven't changed that part of the code for downloads and it WAS working a few days ago... let me try some things and i'll post here when it's fixed.
Can't for the life of me work out why this is happening. It works perfectly on my home local server so I'm going to have to inspect the hosting server to see what the issue is. Weird thing is that my download code is almost identical to the attachment download code so maybe i just need to modify that slightly since attachments work.