title: Why forms.ModelForm loss data from FileField in Django 2.0
body: <p>I have a <code>CustomForm</code> that extends from <code>forms.ModelForm</code> and use a <code>FileField</code> like this:</p>
  
  <pre><code>avatar = forms.FileField(required=False, label="avatar")
  </code></pre>
  
  <p>When I save the model, everything works, and file was saved in directory. But when I re-enter in view (file is over there), and just click in "Save", the file is lost. My <code>views.py</code> is:</p>
  
  <pre><code>def edit_user(request):
      if request.method == "POST":
          form = CustomForm(request.POST, request.FILES)
          if form.is_valid():
              user = form.save(commit=False)
              user.save()
      else:
          user= User\
              .objects\
              .get_or_create(user=request.user,
                             defaults={'name':request.user.first_name,
                                       'email':request.user.email})
          form = CustomForm(instance=user)
  
          return render(request, 'user.html', {'form':form, 'user':user})
  </code></pre>
  
  <p>I'm using <code>Python 3.6</code> and <code>Django 2.0</code>.</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;form method="post" action="{% url 'myprofile' %}"
                                class="form-horizontal" role="form"
                                enctype="multipart/form-data"&gt;
                              {% csrf_token %}
                              {% for field in form %}
                                  &lt;div class="form-group row"&gt;
                                      &lt;label class="col-2 col-form-label"&gt;{{ field.label_tag }}&lt;/label&gt;
                                      &lt;div class="col-10"&gt;
                                          {% render_field field class+="form-control" %}
                                          {% if field.errors %}
                                              &lt;br/&gt;
                                              {% for error in field.errors %}
                                                  &lt;div class="alert alert-danger alert-dismissable"&gt;
                                                      &lt;p&gt;&lt;strong&gt;{{ error|escape }}&lt;/strong&gt;&lt;/p&gt;
                                                  &lt;/div&gt;
  
                                              {% endfor %}
                                          {% endif %}
                                      &lt;/div&gt;
                                      {% if field.help_text %}
                                          &lt;p class="help"&gt;{{ field.help_text|safe }}&lt;/p&gt;
                                      {% endif %}
                                  &lt;/div&gt;
                              {% endfor %}
                          &lt;input type="submit" value="{% trans 'save' %}" &gt;
                          &lt;/form&gt;
  </code></pre>