title: Request.files object is empty on flask server when uploaded using ng-file-upload
body: <p>I am using ng-file-upload to upload a xls or xlsx file, below is my html code    </p>
  
  <pre><code>&lt;div ngf-drop ngf-select ngf-change="checkFile($invalidFiles);" ngf-model-invalid="invalidFiles" ng-model="file" class="center" ngf-drag-over-class="'dragover'" ngf-pattern="'.xls,.xlsx'"&gt;
  
  &lt;button class="some-class" ng-click="upload(file)"&gt;&lt;/button&gt;
  </code></pre>
  
  <p>Angular controller:</p>
  
  <pre><code>$scope.upload = function (file) {
      if (file) {
              var upload = Upload.upload({
                  url: API.url('upload'),
                  data: {
                    file: file
                  },
                  headers:{ // tried passing different headers as well 
                      "Content-Type":"multipart/form-data"
                   }
  
              });
              upload.then(function (resp) {
                  //Some Operation
              }, null, function (evt) {
                  //Some Operation
              });
      }
  };
  </code></pre>
  
  <p>Flask server:</p>
  
  <pre><code>    def upload(self):
          params = self.get_params(request)
          file = request.files['file'] ###This object is empty.
          result = self._upload_file(file)
          return "Dummy"
  </code></pre>
  
  <p>I followed <a href="https://www.tutorialspoint.com/flask/flask_file_uploading.htm" rel="nofollow noreferrer">this</a> documentation for server side. 
  I think the problem is from client side, may be I am sending some bad request as flask server is throwing 400. So I have tried by sending 'multipart/form-data' headers but no luck and also by sending Form data explicitly.
  I have also looked for few solutions like <a href="https://stackoverflow.com/questions/8552675/form-sending-error-flask">sol-1</a>.
  I didn't find anything which could solve my problem in the <a href="https://github.com/danialfarid/ng-file-upload#usage" rel="nofollow noreferrer">ng-file-upload</a> doc as well. </p>
  
  <p>If required I can send the request headers as well.</p>
  
  <p><strong>I might be missing very tiny part, can someone please point me out that.</strong> </p>