Did a little digging and it looks like this may be a Real Player issue.
Real Player changes the Extensions of Windows systems so only Real Player will know these are MP3 files. Disable RealPlayer for handling MP3s on your system. Then download your file again.
Here is another option – When grabbing a mp3 from a site… right click, then choose Save target as… IF the file tries to save itself as *.mpga file at that rename it back to *.mp3
Hopefully one of those will help! If not, someone else jump in here!