The standard user interface: "Open File", "Save To File", "Browse directory".

// File: file.w_js (JScript)

   var hMenu = Window.CreateMenu();
   Window.AppendMenu(hMenu,"MF_STRING",8000,"&Open File");
   Window.AppendMenu(hMenu,"MF_STRING",8001,"&Save To File");
   Window.AppendMenu(hMenu,"MF_STRING",8002,"Select &Directory");
   Window.AppendMenu(hMenu,"MF_SEPARATOR",0,"");
   Window.AppendMenu(hMenu,"MF_STRING",8003,"&Exit");
   Window.AppendMenu(0,"MF_POPUP",hMenu,"File");
   Window.DrawMenuBar();
   Window.DestroyMenu(hMenu);
 


function WM_COMMAND(id)
{ 

// FileDialog(type,str)
// type: 0 - "Open File"
//       1 - "Save To File"
//       2 - "Browse dir"

switch (id)
{ case 8000: 
  var fread = "File to read:";
  fread += Window.FileDialog(0,"default.txt","E:\\","Text(*.txt)|*.txt||");
  Window.TextOut(0,50,50,fread);
  Window.UpdateWindow();
  break;

  case 8001:
  var fsave = "File to save:";
  fsave += Window.FileDialog(1,
                             "default.txt",
                             "E:\\","Text(*.txt)|*.txt|All(*.*)|*.*||");
  Window.TextOut(0,50,80,fsave);
  Window.UpdateWindow();
  break;

  case 8002:
  var dir = "Directory:";
  dir += Window.FileDialog(2,
                          "E:\\Nano",
                          "select folder",
                          "");
  Window.TextOut(0,50,120,dir);
  Window.UpdateWindow();
  break;

  case 8003:
  Window.Close();
  break;
     };
   };

// File: file.w_vbs (VBScript)
   hMenu = Window.CreateMenu
   Window.AppendMenu hMenu,"MF_STRING",8000,"&Open File"
   Window.AppendMenu hMenu,"MF_STRING",8001,"&Save To File"
   Window.AppendMenu hMenu,"MF_STRING",8002,"Select &Directory"
   Window.AppendMenu hMenu,"MF_SEPARATOR",0,""
   Window.AppendMenu hMenu,"MF_STRING",8003,"&Exit"
   Window.AppendMenu 0,"MF_POPUP",hMenu,"File"
   Window.DestroyMenu(hMenu)
   Window.DrawMenuBar

Function WM_COMMAND(id)

' FileDialog(type,str)
' type: 0 - "Open File"
'       1 - "Save To File"
'       2 - "Browse dir"

Select Case id
 Case 8000
  fread = "File to read:"
  fread = fread & Window.FileDialog( 0,"default.txt","E:\\","Text(*.txt)|*.txt||" )
  Window.TextOut 0,50,50,fread
  Window.UpdateWindow

 Case 8001
  fsave = "File to save:"
  fsave = fsave & Window.FileDialog( 1,"default.txt","E:\\","Text(*.txt)|*.txt|All(*.*)|*.*||" )
  Window.TextOut 0,50,80,fsave
  Window.UpdateWindow


 Case 8002
  dir = "Directory:"
  dir = dir & Window.FileDialog( 2,"E:\\Nano","select folder","" )
  Window.TextOut 0,50,120,dir
  Window.UpdateWindow

 Case 8003
  Window.Close
End Select

End Function

Note:

How to create file masks? See this: "description|shablone||" Sample:
"The text files|*.txt|Все файлы|*.*||"