@Catherine: sirbruce and starrigger are right. I did a quick test:
When requesting the ePub-file from your HTTP-Server (Apache 2.2 btw.) it answers with "Content-Type: text/plain" instead of "Content-Type: application/epub+zip". The servers content negotation just doesn't know the mime-type for ePub-files. Please add the correct mime-type "application/epub+zip" for ePub-files (or tell your server administrator to do it).
Quick solution: create a ".htaccess" file in your "downloads" directory on the server containing the line:
Code:
AddType application/epub+zip epub