Django admin custom AdminURLFieldWidget widget misses the Browse button

I'm trying to customize Django admin and I need to create a custom URL displayed for a FileField record in the change_form template. So, by googling I've found the following recipe adpated from a customization of an image field found here. Here is my code:

FileField

change_form

class MyAdminURLFieldWidget(URLInput):
template_name = 'admin/widgets/url.html'

def __init__(self, attrs=None):
final_attrs = {'class': 'vURLField'}
if attrs is not None:
final_attrs.update(attrs)
super(MyAdminURLFieldWidget, self).__init__(attrs=final_attrs)


def get_context(self, name, value, attrs):
context = super(MyAdminURLFieldWidget, self).get_context(name, value, attrs)
context['current_label'] = _('Currently:')
context['change_label'] = _('Change:')
context['widget']['href'] = smart_urlquote('/DownloadView/' + str(value.instance.id) + '/attachment/') if value else ''
return context


class FilesAdmin(admin.ModelAdmin):
list_display = ('id', '_animalid', '_filename', '_filedesc', '_ispublic', 'extra_info')
search_fields = ('subjectid__animalid',)
list_per_page = 50

def formfield_for_dbfield(self, db_field, **kwargs):
if db_field.name == 'filename':
request = kwargs.pop("request", None)
kwargs['widget'] = MyAdminURLFieldWidget
return db_field.formfield(**kwargs)
else:
return super(FilesAdmin, self).formfield_for_dbfield(db_field, **kwargs)


However, the form display the field without the `Browse button:

I would like to have something like the default:

So, how can I make the Browse button to appear in my custom widget?

Browse

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