Hi Pierre, The openDlg method returns an ExtendScript File object, not a string. When you do this:
var fileObj = Open(filename, params, retParm);
filename is a File object instead of a string. You need this instead to get the full path as a string:
var fileObj = Open(filename.fsName, params, retParm);
Or, Russ's method should work as well.
-Rick