FIle upload works in Admin panel but not through ModelForm

[ProPegasus]

I have a model form that lets users upload images. But when we click submit on that form, it's not being uploaded into s3. I have registered the same model in admin and the upload works through admin panel. So I know my s3 has been configured properly. I have made it to return a Httpresponse called error when the form is invalid and it's returning that when I click subit I am very new to django and can't wrap my head around this. Appreciate any help. Thank You

My model

class advert(models.Model):
    category_choices = (
        ("choice", "choice"),
    )
    name = models.CharField(max_length=32)
    part_name = models.CharField(max_length=50, blank=True)
    category = models.CharField(choices=category_choices, max_length=255)
    photo = S3DirectField(dest='primary_destination')
    seller = models.ForeignKey(User, on_delete=models.CASCADE)
    soldby = models.CharField(max_length=32, blank=True)
    place = models.CharField(max_length=32, blank=True)
    cell = models.CharField(max_length=13,blank=True)

My ModelForm

class advertform(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super(advertform, self).__init__(*args, **kwargs)
        self.fields['seller'].widget.attrs['hidden'] = True
    soldby = forms.CharField(widget=forms.TextInput(attrs={'readonly': 'readonly', 'class':'inpt'}))
    place = forms.CharField(widget=forms.TextInput(attrs={'readonly': 'readonly', 'class':'inpt'}))
    cell = forms.CharField(widget=forms.TextInput(attrs={'readonly': 'readonly', 'class':'inpt'}))
    # photo = forms.URLField(widget=S3DirectWidget())
    name = forms.CharField(widget=forms.TextInput(attrs={'class': 'inpt', 'placeholder': 'Name of component'}))
    part_name = forms.CharField(widget=forms.TextInput(attrs={'class': 'inpt', 'placeholder': 'Part no./name'}))
    class Meta:
        model = advert
        fields = ('name', 'part_name', 'category', 'photo', 'soldby', 'place', 'cell', 'seller')

My views function

def addpost(request):
    if not User.is_authenticated:
        return HttpResponse('Please Login')
    else:
        if request.method == 'POST':
            form = advertform(request.POST, request.FILES)
            if form.is_valid():
                advert = form.save()
                advert.refresh_from_db()
                advert.save()
                return HttpResponse('Advertisement added')
            else: return HttpResponse('Error')
        else:
            form = advertform(initial={'soldby': request.user.userprofile.store, 'place': request.user.userprofile.town,'cell': request.user.userprofile.phone,'seller': request.user.id})
        return render(request, 'adpost.html', {'form': form})

HTML

<form class="box2" id="form" method="POST" action="" enctype="multipart/form-data">
              {% csrf_token %}
              {{ form }}
              <button type="submit" id="btn">Publish Ad</button>
</form>