Great idea, but no go. Here’s what I entered into mimes.php, just after XLS. Also, Keynote is presentation software, like Powerpoint.
$mimes = array(
'psd' => 'application/octet-stream',
'pdf' => 'application/pdf',
'swf' => 'application/x-shockwave-flash',
'sit' => 'application/x-stuffit',
'tar' => 'application/x-tar',
'tgz' => 'application/x-tar',
'zip' => 'application/zip',
'gzip' => 'application/x-gzip',
'bmp' => 'image/bmp',
'gif' => 'image/gif',
'jpeg' => 'image/jpeg',
'jpg' => 'image/jpeg',
'jpe' => 'image/jpeg',
'png' => 'image/png',
'txt' => 'text/plain',
'html' => 'text/html',
'doc' => 'application/msword',
'docx' => 'application/msword',
'xl' => 'application/excel',
'xls' => 'application/excel',
'key' => 'application/keynote',
'mov' => 'video/quicktime',
'qt' => 'video/quicktime',
'mpg' => 'video/mpeg',
'mpeg' => 'video/mpeg',
'mp3' => 'audio/mpeg',
'aiff' => 'audio/x-aiff',
'aif' => 'audio/x-aiff',
'aac' => 'audio/aac'
);