Added "known" as a valid model file type
   => 'Known files (*.stl, *.obj, *.amf)|*.stl;*.STL;*.obj;*.OBJ;*.amf;*.AMF;*.xml;*.XML',

When dialog pops up, it can see ALL known file types instead of only stl.
Merge pull request #1059 from cakeller98/Universal_File_Visibility

Added "known" as a valid model file type