Django Form缺少某些FileField的参考

Question

我在Django中有这样的表单：

class ProfessionalForm(forms.ModelForm):
    name = forms.CharField(required=True, label=_("name").title())
    email = forms.EmailField(required=True)
    avatar = forms.FileField(required=False, label=_("avatar").title())
    front_document = forms.FileField(required=False, label=_("front document").title())
    back_document = forms.FileField(required=False, label=_("back document").title())
    class Meta:
        model = Professional
        fields = ("name", "email", "avatar", "front_document", "back_document")

问题是：在页面中，我放了一个文件（例如，“头像”）并保存，一切正常。因此，当我再次在页面中输入并设置另一个文件（例如“ front_document”）时，控制器将丢失对“ avatar”的引用，并将此字段设置为空。发生什么事了？

我的views.py

def myprofile(request):
    if request.method == "POST":
        form = ProfessionalForm(request.POST, request.FILES)
        if form.is_valid():
            professional = form.save(commit=False)
            professional.user = request.user
            professional.save()
            request.session['professional_avatar_url'] = \
                professional.avatar.url if professional.avatar else settings.NO_IMAGE

        else:
            professional_t = Professional \
                .objects \
                .get_or_create(user=request.user,
                               defaults={'name': request.user.first_name + " " + request.user.last_name,
                                         'email': request.user.email})
            professional = professional_t[0]
        return redirect('myprofile')
    else:
        professional_t = Professional\
            .objects\
            .get_or_create(user=request.user,
                           defaults={'name':request.user.first_name + " " + request.user.last_name,
                                     'email':request.user.email})
        professional = professional_t[0]
        form = ProfessionalForm(
            instance=professional)

        return render(request, 'npadmin/myprofile.html', {'form':form, 'professional':professional})

还有我的html：

<form method="post" action="{% url 'myprofile' %}"
                              class="form-horizontal" role="form"
                              enctype="multipart/form-data">
                            {% csrf_token %}
                            {% for field in form %}
                                <div class="form-group row">
                                    <label class="col-2 col-form-label">{{ field.label_tag }}</label>
                                    <div class="col-10">
                                        {% render_field field class+="form-control" %}
                                        {% if field.errors %}
                                            <br/>
                                            {% for error in field.errors %}
                                                <div class="alert alert-danger alert-dismissable">
                                                    <p><strong>{{ error|escape }}</strong></p>
                                                </div>

                                            {% endfor %}
                                        {% endif %}
                                    </div>
                                    {% if field.help_text %}
                                        <p class="help">{{ field.help_text|safe }}</p>
                                    {% endif %}
                                </div>
                            {% endfor %}
                        <input type="submit" value="{% trans 'save' %}" >
                        </form>

我使用Python 3.6和Django 2.0