I've just looked for the explanation about ますた itself, but I unexpectedly I couldn't find any. ～ますた is Internet slang coming from some notorious Internet boards, and has the same meaning as ～ました. Perhaps, the sound し in ました has changed into す, or ます and た have just joined without conjugating ます. But I'm not very sure. Anyway, ～ますた is grammatically wrong, and sounds to fail to pronounce it properly. So, native speakers use it to sound silly or funny.