Hrmm.. Not sure what to say. You probably would need one of the devolopers to answer this.
I did take a look through the attachment.php and it looks like the files that are identified are:
case "pdf": $ctype="application/pdf"; break;
case "exe": $ctype="application/octet-stream"; break;
case "zip": $ctype="application/zip"; break;
case "doc": $ctype="application/msword"; break;
case "xls": $ctype="application/vnd.ms-excel"; break;
case "ppt": $ctype="application/vnd.ms-powerpoint"; break;
case "gif": $ctype="image/gif"; break;
case "png": $ctype="image/png"; break;
case "jpg": $ctype="image/jpg"; break;
So you probably could get away with just:
.exe, .zip, .doc, .xls, .ppt, .gif, .png, .jpg