Add new file type icons

Hi,Seafiler!

Usage scenarios
I’m a GUI designer,we use Seafile to store design files,most of these extension names are .sketch,.psd,.ai.,aep.

But Seahub does not offer these extensions’ thumbnails, it’s difficult for us to identify which is which。

It’s a big deal for us to make custom style for different file types.

Do you mean file type icons or thumbnails?