Accessing the raw file stream from a node-formidable file upload

What you want to do is override the Form.onPart method:

IncomingForm.prototype.onPart = function(part) {
  // this method can be overwritten by the user
  this.handlePart(part);
};

Formidable's default behavior is to write the part to a file. You don't want that. You want to handle the 'part' events to write to the knox download. Start with this:

form.onPart = function(part) {
    if (!part.filename) {
        // let formidable handle all non-file parts
        form.handlePart(part);
        return;
    }

Then open the knox request and handle the raw part events yourself:

part.on('data', function(data) {
    req.write(data);
});
part.on('end', function() {
    req.end();
});
part.on('error', function(err) {
    // handle this too
});

As a bonus, if the req.write(data) return false that means the send buffer is full. You should pause the Formidable parser. When you get a drain event from the Knox stream you should resume Formidable.

Use multiparty instead. It supports this kind of streaming like you want. It even has an example of streaming directly to s3: https://github.com/superjoe30/node-multiparty/blob/master/examples/s3.js