I'm using a dataconection tot submit the form, so i don't need to give a extension. But the file is saved as a xml document. It's not always a problem, it seems only when you sumbit more than one at the same moment (between one minute). May be the connection is still open, en then infopath can't submit or something.
it's very weird, because the name and extension are ok, only the content is deleted. It's an empty document