To get unlimited access to the disk, you will need to set the following tag in the jnlp file:
For this to work, you will also need to sign all the jar files using "jarsigner".
If you don't have a signed jar, you might need to make use of the "JNLP API" to access the file system. (I say might because I'm not sure. You might be able to read a file without a signed jar. I always sign mine.)