static const char *probe_format(mf_t *mf, char *type, enum demux_check check)
{
    if (check > DEMUX_CHECK_REQUEST)
        return NULL;
    char *org_type = type;
    if (!type || !type[0]) {
        char *p = strrchr(mf->names[0], '.');
        if (p)
            type = p + 1;
    }
    for (int i = 0; type2format[i].type; i++) {
        if (type && strcasecmp(type, type2format[i].type) == 0)
            return type2format[i].codec;
    }
    if (check == DEMUX_CHECK_REQUEST) {
        if (!org_type) {
            MP_ERR(mf, "file type was not set! (try --mf-type=ext)\n");
        } else {
            MP_ERR(mf, "--mf-type set to an unknown codec!\n");
        }
    }
    return NULL;
}