A Google search for "Jammes prénom" shows that it is indeed known as a given name in
France, although the name seems to be pretty rare.
There is a little village with the name "Saint-Jammes" in Southern
France:
http://fr.wikipedia.org/wiki/Saint-Jammes
That Wikipedia page says about the etymology of the place name that "Jammes" is an Occitan form of
James.
Rene www.AboutNames.ch