File extensions

On Windows, it is typical that a Save as dialog box will change the extension on the name specified as you change the file types that you are looking for.

Using both the example on the xojo product folder as well as some code in this forum, both ways leave the filename alone with no extension, for example, if I give it a filename of test1 with file types of either csv or txt to choose from.

If the same thing were done in another product (for example Microsoft Office), when I change between csv or txt, the actual file name changes from test1.csv to test1.txt.

How do I get Xojo SaveAsDialog to behave the same way?