Since the files in classes directory of openfire are loaded as resources during plugin execution, you can get access to them by accessing Ressources from class loader.
this can be done by a code like this:
InputStream in = this.getClass().getClassLoader().getResourceAsStream(“your file name”);
OR
InputStream in=XMPPServer.getInstance().getPluginManager().getPluginClassloader(ref to your plugin instance).getResourceAsStream(“your file name”);
both of them worked for me!
so you need to put the file in classes directory of openfire file structure.