Django无法清除上传的后盖

Question

我在django应用中有一个postmodel和一个postform，但是如果我尝试从表单中删除postcover，则会出现以下错误：

  

“ postcover”属性没有与之关联的文件。

我不明白为什么会这样，因为对于附加后字段，它运行良好。

models.py

class Post(models.Model):
    author = models.ForeignKey(User, on_delete=models.CASCADE)
    title = models.CharField(verbose_name="Post Title", max_length=40)
    content = models.TextField(verbose_name="Post Content", max_length=5000)
    tag = models.CharField(verbose_name="Tags/Meta - (sep. by comma)", max_length=50, blank=True)
    category = models.ForeignKey(Category, verbose_name="Category", on_delete=models.CASCADE, null=True)
    postattachment = fields.FileField(
        verbose_name="Post Attachment",
        blank=True,
        null=True,
        upload_to=get_file_path_user_uploads,
        validators=[file_extension_postattachment, file_size_postattachment]
    )
    postcover = fields.ImageField(
                              null=True,
                              blank=True,
                              upload_to=get_file_path_user_uploads,
                              validators=[default_image_size, default_image_file_extension],
                              dependencies=[FileDependency(processor=ImageProcessor(
                                  format='PNG', quality=99, scale={'max_width': 700, 'max_height': 700}))])
    up_vote = models.IntegerField(verbose_name='Post Up-Vote(s)', default=0)
    down_vote = models.IntegerField(verbose_name='Post Down-Vote(s)', default=0)
    published_date = models.DateField(auto_now_add=True, null=True)

forms.py

class PostForm(forms.ModelForm):
    class Meta:
        model = Post
        widgets = {
            'title': forms.TextInput(attrs={'placeholder': 'e.g.: Something about Mr. Nobody'}),
            'tag': forms.TextInput(attrs={
                'placeholder': 'e.g.: April, Cars, Montana'}),
        }
        fields = ['title',
                  'category',
                  'content',
                  'tag',
                  'postcover',
                  'postattachment'
                  ]

    captcha = CaptchaField()

    field_order = ['title',
                   'category',
                   'content',
                   'tag',
                   'postcover',
                   'postattachment',
                   'captcha'
                   ]

    def __init__(self, *args, **kwargs):
        kwargs.setdefault('label_suffix', '')
        super(PostForm, self).__init__(*args, **kwargs)
        self.fields['title'].widget.attrs.update({'class': 'blablablablaaaa'})
...