Downloaded files not working

Hi,

Setting up my store and will sell downloadable products. I have created the product. Then I go to the site and buy it, using the Test Gateway. In Sales Log I approve and I get the mail with the link to the downloadable file. I can then click the link and donwload the file. But then when I try to open it I just get an error message that the file is corrupt.
I have tested the file before uploading and it works great. So it seems like something is happening with the file during or after upload or when downloaded after purchasing it.
I have tested with several different products and files with the same result.
What could be the problem?

Regards
/Madde

https://ww.wp.xz.cn/plugins/wp-e-commerce/