[KELA/GDPR] Study how to test the content of a file before it is uploaded on the server

The purpose of this task is to evaluate if it would be feasible to parse and check the content of a file, after the user has selected it from the file system but before the server uploads it. In another, we would like to execute a java script.

We need to select which event of the rich:fileUpload widget is relevant:
The <rich:fileUpload> component provides a number of specific event attributes:
* The "onadd" event handler called on an add file operation
* The "onupload" which gives you a possibility to cancel the upload at client side
* The "onuploadcomplete" which is called after all files from the list are uploaded
* The "onuploadcanceled" which is called after upload has been canceled via cancel control
* The "onerror" which is called if the file upload was interrupted according to any errors

Then, we want to make sure we will be able to execute a Java Script which will access the content of the document (how ?)