title: How to add custom html in django admin field?
body: <p>Basically, what I want to achieve is to have a media form that show an image  thumbnail after I choose the file in the add or edit interface. I try using this code but it returned in this error.</p>
  
  <pre><code>Unknown field(s) (admin_image) specified for ImageMedia. 
  Check fields/fieldsets/exclude attributes of class ImageMediaAdmin.
  </code></pre>
  
  <p>This is the model class:</p>
  
  <pre><code>class ImageMedia(models.Model):
      caption = models.CharField(max_length=64)
      picture = models.ImageField(upload_to='image_media', blank=True)
  
      def admin_image(self):
          return '&lt;img src="%s" height="300px"/&gt;' % self.picture
      admin_image.allow_tags = True
  
      def __str__(self):
          return self.caption
  </code></pre>
  
  <p>This is the admin class</p>
  
  <pre><code>class ImageMediaAdmin(admin.ModelAdmin):
      list_display = ('caption', 'picture', 'admin_image')
      search_fields = ('caption', 'picture')
      fields = ('caption', 'picture', 'admin_image')
  
  admin.site.register(ImageMedia, ImageMediaAdmin)
  </code></pre>