Hi there, javier.
I'm playing a bit with MyDownloads these days and I might have a solution for your problem.
In /modules/mydownloads/submit.php and /modules/mydownloads/admin/index.php you'll find that XOOPS uses the "formatURL()" function to treat the url (but not the "homepage") field of a submited download. This function, which resides in /include/functions.php, trims extra spaces and adds "http://" to its target strings (only when it doesn't start with "ftp://", "https://", etc).
So you'd have two choices to solve your problem.
1) You'll solve this pretty neatly by editing both submit.php and admin/index.php, replacing
$url = $myts->makeTboxData4Save(formatURL($url));
with
$url = $myts->makeTboxData4Save($url);
for submit.php and doing it twice in /admin/index.php, where you should replace
$homepage = $myts->makeTboxData4Save(formatURL($HTTP_POST_VARS["homepage"]));
WITH
$homepage = $myts->makeTboxData4Save($HTTP_POST_VARS["homepage"]);
AND
$url = $myts->makeTboxData4Save(formatURL($HTTP_POST_VARS["url"]));
WITH
$url = $myts->makeTboxData4Save($HTTP_POST_VARS["url"]);
This obviously makes it much easier to have invalid urls of a kind, namely those with extra spaces, and also makes these fields more versatile, which can be good (mailto: and relative links) or bad (some sort of exploit?).
2) The other way to solve this would be by editing the formatURL() code in functions.php, but that would probably break things beyond our wildest dreams.
. Right 
