window.onload = function() {

    // verifica se é suportado
    if (window.File && window.FileReader && window.FileList && window.Blob) {
        var fileSelected = document.getElementById('txtfiletoread');
        fileSelected.addEventListener('change', function(e) {
            // seta extensao
            var fileExtension = /text.*/;
            var fileTobeRead = fileSelected.files[0];

            // verifica extensao
            if (fileTobeRead.type.match(fileExtension)) {

                var fileReader = new FileReader();
                fileReader.onload = function(e) {
                    var fileContents = document.getElementById('filecontents');
                    fileContents.innerText = fileReader.result;
                }
                fileReader.readAsText(fileTobeRead);
            } else {
                alert("Por favor selecione arquivo texto");
            }

        }, false);
    } else {
        alert("Arquivo(s) não suportado(s)");
    }
}