Saving an image object to a filename without extentions creates an infinite loop

This line of code creates a MapWinGIS.Image from a SnapShot and save it to a new file:

var img = axMap1.SnapShot2(0, axMap1.CurrentZoom, 1000);
var retVal = img.Save(filename);

When filename is d:\snapshot2.jpg everything works but when it has no extention an infinite loop seems to accure and img.Save doesn't come back.