Webform, rename files to a constant name and bypass validation
By looking at the source in /components/file.inc (in Webform module), you will see that the Webform declares a custom element validation callback that performs the check you want to avoid. In order to override this, you will have to hook into the settings form, and provide your own callback for this element.

Below is the code you would use to achieve that. Pay attention to the comments.

function YOUR_MODULE_form_webform_component_edit_form_alter(&amp;$form, &amp;$form_state) {
  // Override the element validation callback for renaming the file uploads.
  if (isset($form['extra']['rename'])) {
    // Loop through the validation callbacks and remove the one we want to
    // override. This way we will not affect any other module that alters this
    // part of the form.
    foreach ($form['extra']['rename']['#element_validate'] as $key =&gt; $validation_callback) {
      if ($validation_callback == '_webform_edit_file_rename_validate') {
        unset($form['extra']['rename']['#element_validate'][$key]);
      }
    }
    // Note: since the original validation callback provided in Webform module
    // only makes sure that the file names are unique, you don't need to provide
    // your own element validation callback. However, I would recommend doing so
    // just in case you have other webform nodes that will need this validation.
    //
    // Add our custom element validation callback.
    $form['extra']['rename']['#element_validate'][] = 'YOUR_MODULE_edit_file_rename_validate';
  }
}


function YOUR_MODULE_edit_file_rename_validate($element, &amp;$form_state, $form) {
  // I recommend still limiting this only to specific nodes, content types, and
  // not disabling completely.
  if ($form['#node']-&gt;nid != YOUR_NODE_ID) {
    $rename = trim($form_state['values']['extra']['rename']);
    form_set_value($element, $rename, $form_state);
    if (strlen($rename) &amp;&amp; !count(token_scan($rename))) {
      form_error($element, t('To create unique file names, use at least one token in the file name pattern.'));
    }
  }
}