I think I solved myself. I changed the content-type of the payloadfile to application/octet-stream and simply included the file contents in the data. Thankfully the data is text and is working fine. Probably would need to change the code a bit to output a binary file.

ls_Method = "POST"
ls_Url    = "https://site.com.au/api/message"
li_FileNum = FileOpen("C:\\TEMP\\test.xml", TextMode!)
FileReadEx(li_FileNum, ls_filedata)
FileClose ( li_FileNum )
loo_xmlhttp = CREATE oleobject
loo_xmlhttp.ConnectToNewObject("Msxml2.XMLHTTP.6.0")
loo_xmlhttp.open (ls_Method, ls_Url, false)
loo_xmlhttp.setRequestHeader("Content-Type", "multipart/form-data; boundary=----WebKitshort")
loo_xmlhttp.setRequestHeader("Authorization", "Bearer " + ls_token)
ls_data = '------WebKitshort~r~nContent-Disposition: form-data; name="payloadFile"; filename="test.xml"~r~nContent-Type: application/octet-stream~r~n~r~n'
ls_data = ls_data + ls_filedata
ls_data = ls_data + '~r~n~r~n------WebKitshort~r~nContent-Disposition: form-data; name="clientUUID"~r~n~r~nTEST-0001~r~n------WebKitshort~r~nContent-Disposition: form-data; name="messageType"~r~n~r~nSubmit.002.00~r~n------WebKitshort--'
loo_xmlhttp.send(ls_data)

Thanks for the response Marco,

The service isn't just going to receive the file. Do you know how i would go about including the other form data with the blob.  EG clientUUID, mesasgeType

ls_data = '------WebKitshort~r~nContent-Disposition: form-data; name="payloadFile"; filename="C:\\temp\\notmuchhere.txt"~r~nContent-Type: text/plain~r~n~r~n~r~n------WebKitshort~r~nContent-Disposition: form-data; name="clientUUID"~r~n~r~nTEST-0001~r~n------WebKitshort~r~nContent-Disposition: form-data; name="messageType"~r~n~r~nSubmit.002.00~r~n------WebKitshort--'

Cheers Michael

Hi Michael,
store the file into a blob variable and send the blob itself:

ll_bloblen = Len(lblb_myinputfile)
...

loo_xmlhttp.setRequestHeader("Content-Length", String(ll_bloblen))
...
loo_xmlhttp.send(lblb_myinputfile)

Cheers,

Marco