Upload attachment with System user token

Hi,
I am trying to upload attachment to UploadFile.ashx with token authentication (Token was obtained for a system user) but the response returned is Unathorized. However when I use my access token the response returned is Success. What can be the issue here with my token authentication please?
(Request url for upload looks like this: https://mysite.tpondemand.com/UploadFile.ashx?token=MYSYSTEMUSERTOKEN)