My site sells download zip files which are being received corrupted

Hi, I have just updated to WP 3.6.1 and WP-ecommerce 3.8.12.1 and now the downloads that I sell are being received by customers corrupted and unopenable. I have tried replacing them all but this hasn’t worked, they open ok in preview on the product page, so it is just happening between them being sold and received by email by the customer. The link used to appear on the screen after purchase too but now it just says the product name and no link.

Please help, I thought I had taken a backup but can’t access it and this is really damaging our business.

Many thanks,