[galaxy-dev] Text field parameter with preset values

When writing a custom tool's xml, is it possible to have a text field with a drop-down list of preset values?

To make this more clear, I would like to have the same effect of the "File Format:" text box of the "Upload File" tool, but allow the user to enter any arbitrary text.

It seems that <param type="select"> only generates a drop-down list that cannot be overridden. <param type="text"> instead does not allow to pre-set some common values.

Any suggestion?

Thanks