I tried with this url:
https://myPath.fogbugz.com/api.asp?cmd=new&sTitle=sometitle&ixProject=1&token=12345678910111213&FileName=C:\Users\John\Desktop\xxx.txt
and the issue is created, but the attachment no. How to do this?
You'll need to use POST rather than GET, POSTing in multipart/form-data format and passing in the files as type "file" and name "File1", "File2", "File3" etc, with a "nFileCount" parameter set to the total number of files being sent.
Have a look at the Editing Cases section in the XML API docs for further details.