Dowloadable files corrupt

I am having trouble with my shop. We sell downloadable files in .zip format and they are arriving at the customers computer corrupted.

It appears as though the free files we offer don’t have this problem or at least no where near as often.

Almost every customer is complaining of receiving corrupt files.

We can send the same file in .zip format to customers via email with no problems, which rules out the creation of the files causing the drama.

We are using 2.0.10. This problem has been apparent since we began in Jan.

Any ideas or hints on where to look.

Thanks Ben.