title: File input in svelte does not update data
body: <p>I'm trying to make a site with client-side file input, where some data is processed entirely on the client and nothing sent to the server.</p>
  
  <p>I have the following Svelte component (App.svelte):</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;script&gt;
  let data;
  function handleFileSelect(evt) {
      var files = evt.target.files;
      if (! (files &amp;&amp; files[0])) {
          return;
      }
      let binfile = files[0];
      let reader = new FileReader();
      reader.onload = function(evt) {
          data = new Uint8Array(evt.target.result);
      }
      reader.readAsArrayBuffer(binfile);
  }
  &lt;/script&gt;
  
  &lt;main&gt;
      &lt;label for="fileinput"&gt;Your data&lt;/label&gt;
      &lt;input type="file" id="fileinput" on:click={handleFileSelect}/&gt;
      &lt;div&gt;
          &lt;textarea id="datafield" rows="10" cols="50"&gt;{data}&lt;/textarea&gt;
      &lt;/div&gt;
  &lt;/main&gt;
  </code></pre>
  
  <p>When I click the button, the data is not updated in the text area, but when I click the button a second time, the data is loaded into the text area as soon as it's clicked. Am I on the right track for loading a file in svelte? What am I missing? Thanks!</p>