When using a QFileDialog::getSaveFileName with automatic file name suffix appending enabled, the overwrite check is done before appending the file name extension, which is wrong.
I have verified that QFileDialog calls the native KDE dialog (Qts own dialog also doesn't provide automatic file name suffix appending).

Steps to reproduce:
1.) create a file called "aaa" (e.g. in the applications path)
2.) open save dialog, e.g. QString fileName = QFileDialog::getSaveFileName(this, "Save", QCoreApplication::applicationDirPath(), "test files(*.zzz)");
3.) enable automatic file name extension appending in the dialog
4.) enter file name "aaa"
5.) confirm saving

=> dialog should return file name "aaa.zzz" (file doesn't exist)
=> instead the user is asked to confirm overwrite of file "aaa" (which already exists)
=> if overwriting is confirmed, file name "aaa" is returned instead of "aaa.zzz"
(if "aaa" didn't exist and the overwrite check passes, "aaa.zzz" is returned as expected)

Used Qt version is 5.7.1.