how to delimit file extension if file uploader

Hi,

does anybody know please, how to delimit the file extension in the File uploader in the form? When I click on the upload button, the Windows Explorer window shows up but eventhough I have restricted the files extensions to jpg|gif|tiff|bmp in Form Wizard, the user sees "All files" and thus can choose any extension. So it allows them to choose e.g. *.txt file and then when they submit the form, the message "Sorry, Your uploaded file type is not allowed" appears. Is it possible to restrict the file extensions right in the browser???

here is that part of the code from the form

<div class="form_item">
  <div class="form_element cf_fileupload">
    <label class="cf_label" style="width: 200px;">Picture 3</label>
    <input class="cf_fileinput cf_inputbox" title="" size="20" id="file_16" name="file_16" type="file" />
    
  </div>
  <div class="cfclear"> </div>

thanks a lot!!!