Use
if( file_exists(plugin_dir_path(__FILE__) . 'myfile.ext') ) {}
instead of
if( file_exists( plugins_url( "myfile.ext", __FILE__ ) ) ) {}
See plugin_dir_path in the Codex.
Use
if( file_exists(plugin_dir_path(__FILE__) . 'myfile.ext') ) {}
instead of
if( file_exists( plugins_url( "myfile.ext", __FILE__ ) ) ) {}
See plugin_dir_path in the Codex.