The $name argument for an admin_setting object is the config setting in which the value is stored. If you pass 'myplugin/mysetting' as the name, you'll be able to read the resulting value by doing get_config('myplugin', 'mysetting'). As far as I can tell by a brief look at the code in admin_setting_configstoredfile.php, this should contain the full path to the saved file.
General developer forum
specific folder for "admin_setting_configstoredfile"
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