Shop Files Not Downloading Correctly Since Update

Hi there
Recently I updated my theme and now it appears that all the products that I have added to my shop (MP3 files) are not downloading correctly since the update. Products added prior to update are downloading correctly When I test purchasing a product and try and download the file it downloads but will not play. When I checked the download there is no file there. This occurs for single MP3 files. If I try to open a zipped file it also won't open but the file name is the one I listed my product as not what the name of the file uploaded to the server is. Very strange!
I uploaded the files via FTP transfer and then used the upload URL option as trying to directly import into the media library fails to install.
Can you please advise how to fix this problem