There are two parts to that problem:
Issue: Default file for Insert Server Behavior set incorrectly
1) When setting either a Static and Dynamic value for the default file setting in the Upload wizard, unless you hit the <Enter> key after setting it, or <Tab> out of that field, the setting will not be retained.
2) In addition, even when this setting is retained during configuration, if no file is uploaded, the default file name is not used for the serverFileName set in the HelperPHP.php file.
It sounds like you have accounted for #1 but not #2