Fix wxFileDialog::GetFilterIndex() when opening files in wxOSX.

Update m_filterIndex in the "opening" case, just as we already did in the
"saving" one (see #13158 and r67550).

Closes #16764.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78346 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/cocoa/filedlg.mm

@@ -694,6 +694,12 @@ void wxFileDialog::ModalFinishedCallback(void* panel, int returnCode)
         {
             panel = oPanel;
             result = wxID_OK;
+
+            if (m_filterChoice)
+            {
+                m_filterIndex = m_filterChoice->GetSelection();
+            }
+
             NSArray* filenames = [oPanel filenames];
             for ( size_t i = 0 ; i < [filenames count] ; ++ i )
             {