add information on valid file names for source code mirroring

[gpunktschmitz]

Describe the change you'd like: when trying to mirror my style from my github project I was unlucky adding my file gpunktstyle.css - searching the github issues of this project I eventually found the valid file names (endings on user.css and user.styl from question #198).

When entering an invalid file name it would be helpful to show the valid file names as hint/error message.