The get_file_name built-in of forms  is currently not implemented  for  Web. 
You could create a Javabean and use it to implement  this  functionality 
 for  WebForms.

Attached below are the source code of JavaBean & the Wrapper  Class   for  the Bean.

CREATING JAVABEAN 
=================

The Bean uses the following methods and Classes
-----------------------------------------------

PropertyChangeSupport  for  the file_name property.

java.awt.FileDialog  class  is used to show the File Dialog.

firePropertyChange method is called to pass a PropertyChange event to all 
classes which are registered as listeners to  this   class .

addPropertyChangeListener is used by other classes to register themself as 
PropertyChangeListener  for   this   class .

 import  java.awt.*;
 import  java.beans.*;
 
   public   class  FDialog  extends  Canvas  {
   private  String file_name ;
   private  PropertyChangeSupport pcs;

 public  FDialog() {
  file_name = "";
  pcs =  new  PropertyChangeSupport( this );
  }

   public   void  set_file_name()
  {
  Frame frame =  new  Frame("Sample");
  java.awt.FileDialog  OpenDlg =  new  FileDialog(frame, "Open ", java.awt.FileDialog.LOAD);
  OpenDlg.setFile("*.*");
  OpenDlg.show();
  String old_file_name = file_name;
  file_name=OpenDlg.getDirectory() + OpenDlg.getFile();
  pcs.firePropertyChange("file_name", old_file_name, file_name);
  }

   public   void  addPropertyChangeListener(PropertyChangeListener pcl)
  {
  pcs.addPropertyChangeListener(pcl);
  }

   public   void  removePropertyChangeListener(PropertyChangeListener pcl)
  {
  pcs.removePropertyChangeListener(pcl);
  }
  }

CREATING WRAPPER  FOR  JAVABEAN 
=============================

The Wrapper  class   implements  the PropertyChangeListener  interface .
It registers itself as a PropertyChangeListener  for  the Bean  class .
The propertyChange method is executed in response to a PropertyChange event.
The propertyChange method dispatches a CustomEvent to the Handler.
 This  event is then trapped in forms using a when-custom-item-event trigger.

 package  oracle.forms.demos;
 import  java.awt.*;
 import  java.awt.event.*;
 import  java.beans.*;
 import  oracle.forms.properties.ID;
 import  oracle.forms.handler.IHandler;
 import  oracle.forms.ui.CustomEvent;
 import  oracle.forms.ui.VBean;

 public   class  FDialogPJC  extends  VBean  implements  PropertyChangeListener{
     private  Component mComp;
     private  IHandler mHandler;
     private   static   final  ID SHOWFILEDIALOG   = ID.registerProperty("showfiledialog");
     private   static   final  ID FILENAME         = ID.registerProperty("filevalue");
     private   static   final  ID FILECHANGEEVENT  = ID.registerProperty("filechangeevent");


   public  FDialogPJC() {
     super ();
     try {

      ClassLoader cl = getClass().getClassLoader();
      Object obj = Beans.instantiate(cl,"oracle.forms.demos.FDialog");
      mComp = (Component)obj;
      mComp.setVisible(true);
      mComp.setEnabled(true);
      ((oracle.forms.demos.FDialog)mComp).addPropertyChangeListener( this );

      // add the component to this 

      add("Center",mComp);
    }
     catch (Exception e){
      e.printStackTrace();
    }
  }

   public   void  init(IHandler handler)
  {
     super .init(handler);
    mHandler = handler;
  }

   public   boolean  setProperty(ID pid, Object value)
  {

     if (pid ==SHOWFILEDIALOG) {
      ((oracle.forms.demos.FDialog)mComp).set_file_name();
       return  true;
                             }
     else 
       return   super .setProperty(pid,value);
  }

   public   void  propertyChange(PropertyChangeEvent pce) {
  String new_file_name=(String)pce.getNewValue();
   try  {
        mHandler.setProperty(FILENAME, new_file_name);
      }
   catch (Exception e) {}
  CustomEvent ce =  new  CustomEvent(mHandler, FILECHANGEEVENT );
  dispatchCustomEvent(ce);
  }

}


Create a Form with a text_item and bean area. The text_item should be named
FILE_NAME and have a length of  255  chars. The Bean_Area should be named
FDIALOG_BEAN and have the implementation  class  set to oracle.forms.demos.FDialogPJC

Code to bring up the FileDialog 
===============================

declare
       FDialog_Bean        Item;
begin
       FDialog_Bean:=find_item('FDIALOG_BEAN');

         /*  FDIALOG_BEAN is the name of the Bean item */
 
       if  NOT id_null(FDialog_Bean) then
           set_custom_item_property(FDialog_Bean,'showfiledialog',  1 );

        /* This invokes the setProperty method of the Bean Wrapper using  
           SHOWFILEDIALOG as ID. The wrapper in turn calls the set_file_name 
           method of the Bean.
        */

      end  if ;
end;


Code to handle the PropertyChangeEvent  ( when-custom-item-event trigger 
======================================              attached to the Bean )

declare

        FDialog_Bean        Item;
        BeanValListHdl ParamList;
	paramType      Number;
	EventName      VarChar2( 20 );
        file_name      varchar2( 255 );
Begin
	FDialog_Bean   := find_item('FDIALOG_BEAN');
	BeanValListHdl := get_parameter_list(:system.Custom_Item_Event_Parameters);
	EventName      := :SYSTEM.Custom_Item_Event;
 
	 if  (EventName = 'filechangeevent') then
	   get_parameter_attr(BeanValListHdl,'filevalue',ParamType, file_name);

          /* file_name variable contains the file_name */
          :file_name := file_name;
        end  if ;
end;