Excel 2007 msoFileDialogSaveAs force ext

That just presets the filter as the user can select any index from the dropdown. You may need to do a check after they make a selection.

Yes, change the dialog type to filepicker and then you can clear out all the filters and add just the one(s) you want.
Dim oDlg As FileDialog
Set oDlg = Application.FileDialog(msoFileDialogFilePicker)
oDlg.Filters.Clear
oDlg.Filters.Add "Excel Macro Enabled Workbooks Only (*.xlsm)", "*.xlsm"
oDlg.FilterIndex = 0
oDlg.Title = "RobDog888's Dialog Filter Demo"
oDlg.Show