Transforming a Webform upload field into a single Attach button
I found these 2 tutorials, with this I can manipulate the upload field only with CSS, without touching the PHP code:

http://jsfiddle.net/gabrieleromanato/mxq9R/

http://geniuscarrier.com/how-to-style-a-html-file-upload-button-in-pure-css/

With this, it creates the link based on an image as background that imitate the form of the button.

Thanks again to @Mołot for helping.