Add an input[type="file"] label
The markup in Drupal is always overridable.  In this case, you may either need to use a hook_form_alter() in a custom module to modify the label html, override the theme_file() theme function (which is responsible for producing the markup for the file input element), or theme the form specifically.  Any of these three methods should be able to meet your needs (although I'm partial to the theme_file() approach).