Sign Request with file as request body

I’m working on an application that allows a user to select a file using the standard <input type="file"> on the front-end (using React). I then sign my POST request using this aws4 module, however, that module (and I believe signature v4 in general) can only accept and sign strings or a binary buffer as the body of a request. Once I retrieve the file object from the input form, how would I go about correctly signing and sending my file to my API Gateway server?