Вот один из способов:
HTML
<tr>
<td>Select a File to Load:</td>
<td><input type="file" id="fileToLoad"></td>
<td><button onclick="loadFileAsText()">Load Selected File</button><td>
</tr>
JavaScript
function loadFileAsText(){
var fileToLoad = document.getElementById("fileToLoad").files[0];
var fileReader = new FileReader();
fileReader.onload = function(fileLoadedEvent){
var textFromFileLoaded = fileLoadedEvent.target.result;
document.getElementById("inputTextToSave").value = textFromFileLoaded;
};
fileReader.readAsText(fileToLoad, "UTF-8");
}