I have a form in my html page, that prompts user to upload File or Image to the server. I want to be able to upload ether file or image. Let's say if user choose file, image should be null, and vice verso. Right now I can only upload both of them, without error. But If I choose to upload only one of them (let's say I choose image) I will get an error:

"Key 'attachment' not found in <MultiValueDict: {u'image': [<InMemoryUploadedFile: police.jpg (image/jpeg)>]}>"

models.py:

#Description of the file 
class FileDescription(models.Model):

    TYPE_CHOICES = (
        ('homework', 'Homework'),
        ('class', 'Class Papers'),
        ('random', 'Random Papers')                    
    )

    subject = models.ForeignKey('Subjects', null=True, blank=True)
    subject_name = models.CharField(max_length=100, unique=False)

    category = models.CharField(max_length=100, unique=False, blank=True, null=True)

    file_type = models.CharField(max_length=100, choices=TYPE_CHOICES, unique=False) 
    file_uploaded_by = models.CharField(max_length=100, unique=False)
    file_name = models.CharField(max_length=100, unique=False)
    file_description = models.TextField(unique=False, blank=True, null=True)
    file_creation_time = models.DateTimeField(editable=False)
    file_modified_time = models.DateTimeField()

    attachment = models.FileField(upload_to='files', blank=True, null=True, max_length=255)
    image = models.ImageField(upload_to='files', blank=True, null=True, max_length=255)

    def __unicode__(self):
        return u'%s' % (self.file_name)

    def get_fields(self):
        return [(field, field.value_to_string(self)) for field in FileDescription._meta.fields]

    def filename(self):
        return os.path.basename(self.image.name)

    def category_update(self):
        category = self.file_name
        return category

    def save(self, *args, **kwargs):
        if self.category is None:
            self.category = FileDescription.category_update(self)
        for field in self._meta.fields:
            if field.name == 'image' or field.name == 'attachment':
                field.upload_to = 'files/%s/%s/' % (self.file_uploaded_by, self.file_type)
        if not self.id:
            self.file_creation_time = datetime.now()
        self.file_modified_time = datetime.now()
        super(FileDescription, self).save(*args, **kwargs)

forms.py

class ContentForm(forms.ModelForm):
    file_name =forms.CharField(max_length=255, widget=forms.TextInput(attrs={'size':20}))
    file_description = forms.CharField(widget=forms.Textarea(attrs={'rows':4, 'cols':25}))

    class Meta:
        model = FileDescription 
        exclude = ('subject',
                   'subject_name',
                   'file_uploaded_by',
                   'file_creation_time',
                   'file_modified_time',
                   'vote')

    def clean_file_name(self):
        name = self.cleaned_data['file_name']
        # check the length of the file name
        if len(name) < 2:
            raise forms.ValidationError('File name is too short')
        # check if file with same name is already exists
        if FileDescription.objects.filter(file_name = name).exists():
            raise forms.ValidationError('File with this name already exists')
        else:
            return name

views.py

if request.method == "POST":
        if "upload-b" in request.POST:
            form = ContentForm(request.POST, request.FILES, instance=subject_id)       
            if form.is_valid(): # need to add some clean functions
               # handle_uploaded_file(request.FILES['attachment'],
               #                      request.user.username,
               #                      request.POST['file_type'])
                form.save()
                up_f = FileDescription.objects.get_or_create(
                    subject=subject_id,
                    subject_name=subject_name,
                    category = request.POST['category'],
                    file_type=request.POST['file_type'],
                    file_uploaded_by = username,
                    file_name=form.cleaned_data['file_name'],
                    file_description=request.POST['file_description'],
                    image = request.FILES['image'],
                    attachment = request.FILES['attachment'],
                )
                return HttpResponseRedirect(".")