I think you meant beside the file select/explorer button. You can’t remove or edit this via WP plugin, because it’s browser-dependent. You’d have to hack the browsers.
If you view your form in the various browsers, you’ll see that Chrome says “No file chosen”, FF says “No file selected”, and in IE it’s blank. In FF and IE, the attachment button label is “Browse”, while in Chrome it’s “Choose File”.
On second thought, and I haven’t tried this, but you may be able to mask the text using a borderless div layer with background-color that matches your page theme. You may be able to pull this off via table cells. Do let us know if it works.
