Make it easier to find out how to get a file's extension

Apparently, the way to get a file extension with Qt is to use http://doc.qt.io/qt-5/qfileinfo.html#suffix and http://doc.qt.io/qt-5/qfileinfo.html#completeSuffix. Personally, I would never think to search for these terms when looking for a way to get a file's extension. We should mention "extension" in the documentation for both of these functions.