Opens the standard Open File dialog so that the user could select files for upload. In other words it works in the same manner as if the Add Files button were clicked.
getImageUploader("ImageUploaderID").AddFiles();
Unlike in the file dialog opened by the browser for the <input type="file">
element, multiple selection is allowed.
The file filter for the dialog is formed from the FileMask property.
This method works only if the PaneLayout property is set to the OnePane. For other layouts this method do
nothing.