django model form RegexValidator error message not showing up - django

Question

I'm using the class below in the models.py to restrict file uploads to certain file extensions.
class ExtensionValidator(RegexValidator):
def __init__(self, extensions, message=None):
if not hasattr(extensions, '__iter__'):
extensions = [extensions]
regex = '\.(%s)$' % '|'.join(extensions)
if message is None:
message = 'File type not supported. Accepted types are: %s.' % ', '.join(extensions)
super(ExtensionValidator, self).__init__(regex, message)
def __call__(self, value):
super(ExtensionValidator, self).__call__(value.name)
Here is the model form where it is called:
class Product(models.Model):
user = models.ForeignKey(User)
name = models.CharField(max_length=100)
description = models.CharField(max_length=300)
price = models.DecimalField(max_digits=10, decimal_places=2)
url = models.CharField(max_length=200, blank=True)
product_type = models.CharField(max_length=15, choices=product_types, default='choose')
image = models.ImageField(upload_to='product_images', blank=True, null=True)
image_url = models.CharField(max_length=200, blank=True)
product_file = models.FileField(upload_to='product_files', validators=[ExtensionValidator(['jpg', 'jpeg', 'png', 'zip'])], blank=True, null=True)
likes = models.IntegerField(default=0)
def __str__(self):
return self.name
def get_absolute_url(self):
return reverse('index', kwargs={})
I've tested and the functionality works, but the error message is not showing up when the user submits the form. For all forms I'm using an include - form.html below:
{% load widget_tweaks %}
{% for hidden_field in form.hidden_fields %}
{{ hidden_field }}
{% endfor %}
{% if form.non_field_errors %}
<div class="alert alert-danger" role="alert">
{% for error in form.non_field_errors %}
{{ error }}
{% endfor %}
</div>
{% endif %}
{% for field in form.visible_fields %}
<div class="form-group">
{{ field.label_tag }}
{% if form.is_bound %}
{% if field.errors %}
{% render_field field class="form-control is-invalid" %}
{% for error in field.errors %}
<div class="invalid-feedback">
{{ error }}
</div>
{% endfor %}
{% else %}
{% render_field field class="form-control is-valid" %}
{% endif %}
{% else %}
{% render_field field class="form-control" %}
{% endif %}
{% if field.help_text %}
<small class="form-text text-muted">{{ field.help_text }}</small>
{% endif %}
</div>
{% endfor %}
I'm guessing I need to add something to the form.html to display the error message, just not sure what or where, yet. I'm still researching this, but any guidance would be gratefully received.
EDIT:
As requested, here's the form class:
class ProductForm(forms.ModelForm):
class Meta:
model = Product
fields = ['name', 'description', 'url', 'product_type', 'price', 'image', 'image_url', 'product_file']
labels = {
'name': 'Product Name',
'url': 'Product URL',
'product_type': 'Product Type',
'description': 'Product Description',
'image': 'Product Image',
'image_url': 'Product Image URL',
'price': 'Product Price',
'product_file': 'Product File',
}
widgets = {
'description': Textarea(attrs={'rows': 5}),
}
EDIT 2:
...and here's the HTML:
<div class="product card card-default">
<div class="card-header">
<h3 class="card-title mb-4">Add Product</h3>
</div>
<div class="card-body">
<form enctype="multipart/form-data" action="post_url/" method="post" >
{% csrf_token %}
{% include 'includes/form.html' with form=form %}
<input class="btn btn-product" type="submit" value="Upload" />
</form>
</div>
</div>