We all know and appreciate .qml files. Some of us for shipping QGIS
styles some of us for writing QtQuick applications .
The second one become more important with Qt5 and so increase the risk
of confusing the two file types. We already have technical collisions of
the two file endings now 
To avoid this, I propose to rename .qml files to .qsl for QGIS 3.
Qgis-developer mailing list
List info: http://lists.osgeo.org/mailman/listinfo/qgis-developer