thanks AaronArmagost
in fact i have solved (almost) most of it.
when you drag and drop any file , it is uploaded to a new drive on the remote machine , called (X on Guacamole RDP) where x is a free drive letter . then the client can copy the uploaded files from it into the remote server ,,,
and when the user need to have files from the remote server to his machine , there is a folder inside that drive called download , as soon the user paste on it, it is downloaded on this browser and he should choose where to save in on his pc.
it turn out that some Windows 2008 servers (not all) does not show this drive letter so no place to upload or download files, i guess it is some sort of security at the client , but still ii cannot figure it out.