Theming a webform file upload widget
It is more likely a Javascript / jQuery related questions rather then Drupal.

The button label and text is defined by the browser which you cannot tweak in Drupal. But with Javascript / jQuery, you can override and take over event for creating your own file upload button.

Since jQuery is included in Drupal, a jQuery library should be suitable for you. jQuery File Upload is one of the best library for this purpose.

I found there are 2 contrib modules. They are jQuery File Upload and jQuery Upload FAPI.