I don't think this is going to be possible without some fairly ugly CSS hacking. The file input box is there for a good reason: security. If it could be controlled (or removed) using JavaScript, users could potentially have sensitive files uploaded without their knowledge or consent.
The input file box is a way of them having some confidence that what they are uploading is only the file they actually want to upload.
The only thing I have seen done successfully with input file boxes is styling them - have a look at this and this for examples.