title: This field is required. Error with ImageField and FileField on Django
body: <p>I am new on Django, I am using Django 1.6.1 and in one of my form I get the next error </p>
  
  <pre><code> This field is required.
  </code></pre>
  
  <p>I don't understand why I get this error, my form is very basic, so my Model, the statement who launch this error is form.is_valid(), but I don't know why. 
  Some help would be apreciate.</p>
  
  <p>my views is:</p>
  
  <pre><code>if request.method == 'POST':
      form = campoImagenForm(request.POST, request.FILES)
      if form.is_valid():
          articulo = form.save(commit=False)
          articulo.item = item
          articulo.atributo = atributo
          articulo.save()
          return HttpResponseRedirect('/workproject/' + str(project.id))
  </code></pre>
  
  <p>my Form is:</p>
  
  <pre><code>class campoImagenForm(forms.ModelForm):
      class Meta:
          model = campoImagen
          fields = ('imagen',)
  </code></pre>
  
  <p>my Model is: </p>
  
  <pre><code>class campoImagen(models.Model):
      item = models.ForeignKey(ItemBase)
      atributo = models.ForeignKey(TipoItem)
      imagen = models.ImageField(verbose_name='Imagen', upload_to='archivos')
      version = models.IntegerField()
  </code></pre>
  
  <p>I don't have a clue of why I get such an error. </p>