Intermediate solution:
w600tool.py modificated to have fitting Stringoutput as download.py
and file “upload.bat” changed to use w600tool.py-script for upload.
Works for me ! 
*:\Users\js\AppData\Local\Arduino15\packages\w600\tools\wmtools\0.3.2/upload.bat C:\Users\js\AppData\Local\Arduino15\packages\w600\tools\wmtools\0.3.2 COM15 115200 C:\Users\js\AppData\Local\Temp\arduino_build_131628/sketch_apr16a.ino.gz.img *
Use python script to upgrade the firmware.
python C:\Users\js\AppData\Local\Arduino15\packages\w600\tools\wmtools\0.3.2/w600tool.py -p COM15 --upload C:\Users\js\AppData\Local\Temp\arduino_build_131628/sketch_apr16a.ino.gz.img --upload-baud 115200
Opening device: COM15
Uploading C:\Users\js\AppData\Local\Temp\arduino_build_131628/sketch_apr16a.ino.gz.img
*Start upgrade C:\Users\js\AppData\Local\Temp\arduino_build_131628/sketch_apr16a.ino.gz.img *
Please wait for upgrade …
Upgrade C:\Users\js\AppData\Local\Temp\arduino_build_131628/sketch_apr16a.ino.gz.img image success!
Please wait for about 10 seconds before uncompress & restart…
Replace in upload.bat:
python %tools_path%/download.py -c %console% -b %baud_speed% -f %img_file%
With:
python %tools_path%\w600tool.py -p %console% –upload %img_file% –upload-baud %baud_speed%
w600tool.py you will find by google…
Workaround
Press Reset before Upload!
Use python script to upgrade the firmware.
python C:\Users\js\AppData\Local\Arduino15\packages\w600\tools\wmtools\0.3.2/w600tool.py -p COM15 --upload C:\Users\js\AppData\Local\Temp\arduino_build_131628/sketch_apr16a.ino.gz.img --upload-baud 115200
Opening device: COM15
Uploading C:\Users\js\AppData\Local\Temp\arduino_build_131628/sketch_apr16a.ino.gz.img
*Start upgrade C:\Users\js\AppData\Local\Temp\arduino_build_131628/sketch_apr16a.ino.gz.img *
Please wait for upgrade …
Upgrade image success! => C:\Users\js\AppData\Local\Temp\arduino_build_131628/sketch_apr16a.ino.gz.img
Please wait for about 10 seconds before uncompress & restart…
regards,
Juergen