[Mordekay]

In 99% of all cases it is a spelling problem.

Open the .sma and check that the complete path to the file and it's name is EXACTLY written the same like on your web-server. If you are on a Linux server it is case sensitive. To prevent problems make sure all letters are written small. Remove spaces in names, that can also cause problems.
If you add the path of the file and it's name to your sv_downloadurl and open it in a browser you must be asked to download the file., if not then there is something wrong with the path to the file/filename.