title: Image url from an ImageField used as ForeignKey
body: <p>I'm trying to see an image inside a template using this tag {{ post_details.header_image.url }}. When I do this the image doesn't appear.</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>class ImageUpload(models.Model):
      name = models.CharField(
          max_length=50,
          )
      file = models.ImageField(
          upload_to='images/%Y/%m/%d',
          )
  
  class BlogPost(models.Model):
      title = models.CharField(
          max_length=70,
          unique=True,
          )
      slug_post = models.SlugField(
          'Slug',
          max_length=70,
          unique=True,
          )
      contents = models.TextField(
          'Contenuti',
          blank=True,
          )
      header_image = models.ForeignKey(
          ImageUpload,
          on_delete=models.CASCADE,
          related_name="blog_header_image",
          blank=True,
          )
  </code></pre>
  
  <p><strong>views.py</strong></p>
  
  <pre><code>def singlePost(request, slug_post):
      post_details = get_object_or_404(BlogPost, slug_post=slug_post)
      context = {
          "post_details": post_details,
          }
      template = 'blog/reading/single_post.html'
      return render(request, template, context)
  </code></pre>
  
  <p><strong>single_post.html</strong></p>
  
  <pre><code>  &lt;img src="{{ post_details.header_image.url }}" class="img-fluid" alt="{{ post_details.title }}"&gt;
    &lt;h1&gt;{{ post_details.title }}&lt;/h1&gt;
    &lt;p&gt;{{ post_details.contents }}&lt;/p&gt;
  </code></pre>
  
  <p>I can see the title and the contents but the image tag appear blank.
  How I can solve this problem?</p>