Applet - get data from html form element with JSObject

Hello,

I have problem with applet which read data from HTML form, code reading the data looks like this:

JSObject browserWindow = JSObject.getWindow(signerApplet);
JSObject inputField = (JSObject) browserWindow.eval("document.getElementById('" + getDataInputField() + "')");
String data = (String) inputField.getMember("value");

This solution works, until incoming data is too big. In case the data amount is bigger than cca 10Mb, method getMember returns only null. But there isn't thrown any exception, and also nothing goes to console.
I have also tried to increase java memory in control panel, but it doesn't help.

I am trying to figure out what is wrong here for several days, and i am out of ideas.
If somebody knows how to solve problem with JSObject and getmember method, or if there is some other way to get data from the html form, i would be very thankful.