google app engine django model form无法获取blobproperty

我有以下型号:

class Image(db.Model):
    auction = db.ReferenceProperty(Auction)
    image = db.BlobProperty()
    thumb = db.BlobProperty()
    caption = db.StringProperty()
    item_to_tag = db.StringProperty()

以及以下形式:

class ImageForm(djangoforms.ModelForm):
    class Meta:
        model = Image

当我调用imageForm()时,只创建非blob字段,如下所示:

<tr><th><label for="id_auction">Auction:</label></th><td><select name="auction" id="id_auction">
<option value="" selected="selected">---------</option>
<option value="ahRoYXJ0bWFuYXVjdGlvbmVlcmluZ3INCxIHQXVjdGlvbhgKDA">2010-06-19 11:00:00</option>
</select></td></tr>
<tr><th><label for="id_caption">Caption:</label></th><td><input type="text" name="caption" id="id_caption" /></td></tr>
<tr><th><label for="id_item_to_tag">Item to tag:</label></th><td><input type="text" name="item_to_tag" id="id_item_to_tag" /></td></tr>

我希望BLOB字段也包括在表单中(以及文件输入)。我做错什么了?