botón para abrir un archivo local desde html

html
<input type="file" id="fileinput" />
<script>
    document.getElementById('fileinput').addEventListener('change', readSingleFile, false);
    function readSingleFile(evt) {
        var f = evt.target.files[0]; 
        if (f) {
            var r = new FileReader();
            r.onload = function(e) { 
                var contents = e.target.result;
                alert( "Got the file.n" 
                      +"name: " + f.name + "n"
                      +"type: " + f.type + "n"
                      +"size: " + f.size + " bytesn"
                      + "starts with: " + contents.substr(1, contents.indexOf("n"))
                );  
            }
            r.readAsText(f);
        } else { 
            alert("Failed to load file");
        }
    }
</script>