title: How to use ng2-file-upload to post a request without the file?
body: <p>I come across a question about ng2-file-upload, If I use it to upload at least a file, it works well. But if I do not select any file, it will crash and reload, and I can't get any error. Anyone know how to use ng2-file-upload to post a request without the file? 
  This is my code:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
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  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div&gt;
    &lt;span&gt;Upload：&lt;/span&gt;
    &lt;input type="file" ng2FileSelect [uploader]="uploader" (change)="selectedFileOnChanged($event)"/&gt;
  &lt;/div&gt;</code></pre>
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  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>uploader:FileUploader = new FileUploader({    
      url: "http://www.download.com:80/uploadFile",   
      method: "POST",    
      itemAlias: "uploadedfile"
  });
  this.uploader.uploadAll();</code></pre>
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