Temporary downloaded file name bug fix (Bug report #122)

Hello, 

I made this request because I thought it would be nice to share this personal patch. Even if I know that there are cleaner to fix bug # 122. But I also try to learn how to use git and github, so I thought ... what better than a concrete example to get into all that. And since i'm learning python... i had many good reasons to this pull request. There is also the fact that i really like mpsyt, i use it alot ! So i wanted to contribute.

I would like to discuss about what would be most appropriate way to fix this bug ?

Thank You