CC3220S-LAUNCHXL: how to rename a file to "/sys/mcuimg.bin", i do this but it will appear error SL_ERROR_FS_FILE_NAME_RESERVED

Part Number:CC3220S-LAUNCHXL

I'm doing a boot-loader myself, I load all mcu data named as "/sys/mcuimg_read.bin", and then delete "/sys/mcuimg.bin" and rename "/sys/mcuimg_read.bin" to "/sys/mcuimg.bin", it return me error SL_ERROR_FS_FILE_NAME_RESERVED

    char*           DeviceFileName_mcuimg_read = "/sys/mcuimg_read.bin"; 

char* NewFileName = "/sys/mcuimg.bin";
RetVal = sl_FsCtl(SL_FS_CTL_RENAME, MasterToken, (unsigned char *)DeviceFileName_mcuimg_read,
(unsigned char *)NewFileName, 0, NULL, 0, NULL );