The standard pluginfile.php script (in the root directory of your Moodle install) that is the usual entry point for serving files, will check to see which plugin the file belongs to, then open up the lib.php script from that plugin and hand over to it to see if the file should be served or not. If lib.php does not exist, or does not contain a PLUGINNAME_pluginfile() function, then it assumes that plugin does not serve any files, so will just return a 'file not found' error.
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