ActiveX .NET Drag&Drop control work fine in windows form not in web Form

Question

I have create an activex Control with Visual studio 2005 and framework
2.0 in c# to add drag & drop functionality to upload multi file.
 When I use it in a windows form it work fine. Infact if I select 3
files from a windows explorer and I move it in a listbox in my
activeX, I find my 3 files added in the listbox but if I use my
activeX in a web form and  I select 3 file from a windows explorer in
the listBox find only 1 file the file on which the mouse is on when
the drag & drop operation begin.

I post some code..

 private void listBox1_DragDrop(object sender, DragEventArgs e)
       {
                Array a = (Array)e.Data.GetData(DataFormats.FileDrop);

               if (a != null)
               {
                  // if i execute the activeX in a windows Form the
a.Length property value is the exact number of file that i have
selected
                 //  if i execute the activeX in a web Form the
a.Length property value is always 1                    for (int i =0 ;
i< a.Length; i++)
                   {                                         string s
= a.GetValue(i).ToString();

                       if (!listBox1.Items.Contains(s))
                           {
                               string filename =
System.IO.Path.GetFileName(s);
                               listBox1.Items.Add(filename);
                                             }
                   }
               }
   }

 Anyone knows why in web Form The value of a.Length is Always 1?

Thanks wery much