HTTP Parameter Error in method SET_FORM_FIELD

Hi,

I have to use a web service with 3 obligatory parameters . User , password and File.

I have no problems with the two first parameters passing them with the class

cl_http_client method ->request->set_form_field. and this values.

However i cant pass the third parameter (File) using the same mode.

i tried:

CALL METHOD http_client->request->set_form_field

EXPORTING

name = 'file'

value = 'Path source of my file'.

but i receive an error response from the web service .

Thank.