$("input:file").change(function(e){
    var input=e.target;

    // create a new FileReader object, and …
    var reader=new FileReader();

    // … tell it what to do in case of it successfully loading the file
    reader.onloadend=function(e){
        var fileBase64=e.target.result;
        $(input).siblings("input:text").first().val(fileBase64);
    };

    // … and have it load the file
    reader.readAsDataURL(input.files[0]);
});