Private Download problem

hi, i've a little problem when i want to download an attachement. I explain this... my drupal download method is Private and the problem is simple, file name change during the download process. Drupal send me a correct file but worng name...
Example:

File upload = ubuntu.rar
File download = x3n4xt4us

if i rename the download file as ubuntu.rar, this one works correctly.

What can I do?

thanks...