attachment_fu content_type is blank for zip files

Hi,

I am using attachment_fu plugin. Most if it is working fine, except when I upload a .zip (or .rar) file, the content_type is blank. If I turn the validation on, this upload will fail. When I turn validation off, I see in the database the content_type is blank for the uploaded file.

Can anyone point me in the right direction on how I can fix this?

Thanks,

Changpeng