Jezze

I was able to implement the feature in your plugin. I changed the function wpuxss_eml_post_mime_types() in mime-types.php to this.

function wpuxss_eml_post_mime_types( $post_mime_types )
{
  $wpuxss_eml_mimes = get_option('wpuxss_eml_mimes');

  if ( !empty($wpuxss_eml_mimes) )
  {
    $mineGroups = array();
    foreach ( $wpuxss_eml_mimes as $type => $mime )
    {
      if ( $mime['filter'] == 1 )
      {
        $key = $mime['singular'];
        if ( !isset($mineGroups[$key]) )
        {
          $mineGroups[$key] = array(
              'singular' => $mime['singular'],
              'plural' => $mime['plural'],
              'mime' => $mime['mime'],
          );
        }
        else
        {
          $mineGroups[$key]['mime'] .= ',' . $mime['mime'];
        }
      }
    }

    if ( !empty($mineGroups) )
    {
      foreach ( $mineGroups as $type => $mineGroup )
      {
        $post_mime_types[$mineGroup['mime']] = array(
          __($mineGroup['singular']),
          __('Manage ' . $mineGroup['singular']),
          _n_noop($mineGroup['singular'] . ' <span class="count">(%s)</span>', $mineGroup['plural'] . ' <span class="count">(%s)</span>')
        );
      }
    }
  }

  return $post_mime_types;
}

Basically it creates comma separated MIME-type keys in $post_mime_types, which should work according to this blog http://ageekandhisblog.com/wordpress-how-to-add-more-upload-categories-filter/

But unfortunately, there seems to be a bug (?) in /wp-admin/includes/class-wp-media-list-table.php in function get_views() and I had to change a few lines, which would be a quite bad requirement for the plugin. Anyway, I changed the following lines.

$matches = wp_match_mime_types(array_keys($post_mime_types), array_keys($_num_posts));

to

$post_mime_types_piped = array_keys($post_mime_types);
foreach ($post_mime_types_piped as $key => $value)
  $post_mime_types_piped[$key] = str_replace(",", "|", $value);

$matches = wp_match_mime_types($post_mime_types_piped, array_keys($_num_posts));

and

if ( !empty( $num_posts[$mime_type] ) )
  $type_links[$mime_type] = "<a href='upload.php?post_mime_type=$mime_type'$class>" . sprintf( translate_nooped_plural( $label[2], $num_posts[$mime_type] ), number_format_i18n( $num_posts[$mime_type] )) . '</a>';

to

$mime_type_piped = str_replace(',', '|', $mime_type);

if ( !empty( $num_posts[$mime_type_piped] ) )
  $type_links[$mime_type] = "<a href='upload.php?post_mime_type=$mime_type'$class>" . sprintf( translate_nooped_plural( $label[2], $num_posts[$mime_type_piped] ), number_format_i18n( $num_posts[$mime_type_piped] )) . '</a>';

I hope this will help you.

Some additional info.

My blog/bbpress has nearly than 3000 users and for each of them the following query will be executed every time the website is loaded.

SELECT user_id, meta_key, meta_value
FROM wp_usermeta
WHERE user_id IN (<user_id>)