I don't quite understand this part of your post:
"the problem is: I always get the path to the file on the server side and I could found the file on HP QC?"
The property name is ServerFileName. The definition is:
The path to the attachment on the server machine
So, you are getting the correct value if you are getting a path to the file on the HP QC server. On the system where you are executing the code, the user that you are logged on to the system as would need to have direct access to the HP QC Server in order to access the file using that path, assuming that path makes sense from the system where you run the code.
You might want to look at using the Load method of the Attachment object to first download the attachment to your local system where you are running the code. Then attempt to access the local copy rather than the original file on the server.