I know the $downloadfile - and I want the $user_id. By trial and error I found that this does what I want. But it's 3 separate queries and 3 while loops. I have a feeling there is a better way. And yes, I only have a very little idea about what I'm doing :)

$result = pod_query("SELECT ID FROM wp_posts WHERE guid LIKE '%/$downloadfile'");
while ($row = mysql_fetch_assoc($result)) { 
    $attachment = $row['ID']; }

$result = pod_query("SELECT pod_id FROM wp_pods_rel WHERE tbl_row_id = '$attachment'");
while ($row = mysql_fetch_assoc($result)) {
    $pod_id = $row['pod_id']; }

$result = pod_query("SELECT tbl_row_id FROM wp_pods_rel WHERE tbl_row_id = '$pod_id' AND field_id = '28'");
while ($row = mysql_fetch_assoc($result)) {
    $user_id = $row['pod_id']; }