The <input type="file">
WILL look different in different browsers. This is the way they and we have to live with it.
But I don't think this is something to really worry about. If someone chooses to work with a browser, they decide the work with the browser's look and feel, which includes the rendering of <input type="file">.
It is also not possible to change the button styles of this control (in a decent/clean manner).
In my opinion you can leave them the way they are. For me the <input type="file">
is some sort of a system control owned by the system (the browser), and not the programmer. You cannot also decide which files to display and what not to (eg., filter and display ".doc" and ".docx" and nothing else is not possible)
If for some reason you really really need a single uniform interface: some websites use a picture of a button and use flash to display the file open box. This relies on the presence of flash though. I remember (2 years old memory) there was a control in YUI for this kind of file input (based on flash), perhaps there is a similar one in GWT. I am not very sure though.