Hello,
basically in the same way like you would do it in a "non-browser-compatible" form.
Please, have a look here for more details:
http://office.microsoft.com/en-us/infopath/HP100809081033.aspx
Please be aware that if you are designing a browser-compatible form template, you won't be able to specify which file types are allowed in the file attachment control.
Hope that helps.
Regards, Karsten