Is it possible to add additional controls to a file selection dialog? If not, what other ways are there to present load/save options to the user?
In wx 2.9 there is wxFileDialog::SetExtraControlCreator(), which allows adding additional controls at least on some platforms.
As some kind of a workaround in wx 2.8, one could use the wildcard/filter field and list the same file type multiple times with different description, for example "Plain text, short format", "Plain text, very verbose format".