Fix/sd file list

OctoPrint does not filter the file list returned from M20 in any way, which makes sense for other firmwares where only gcode files are present. However, since Smoothie is a mass storage device and the uploads folder all at once, it contains config.yaml, Smoothie's config file, and often other cruft, so this will make OctoPrint ignore any file name returned in M20 that does not contain '.g'.