Java applet can't access non-public file on same webserver
===

I'm creating a Java Applet for a customer, and I've run into a problem where I need to read/write to a file on their web server, but that file can not, under any circumstances, be publicly accessible. 

The Java Applet and the file are both on the same server, and I can put the file and the .jar file in the same folder if need be, but I've tried several different configurations including having them in the same folder and accessing the file with 

    test = WorkbookFactory.create(new File("test.xlsx"))

I've tried writing out the full path to the file, and nothing seems to work.

The error I'm getting is:

    Caused by: java.security.AccessControlException: access denied ("java.io.FilePermission" "test.xlsx" "read")


What would be the best way to read/write to a file on the same server, that can't be seen publicly?