Django snippet: reduce image size during upload

Just a quick python snippet I written today to reduce the image size during upload

class PhotoField(forms.FileField, object):

    def __init__(self, *args, **kwargs):
        super(PhotoField, self).__init__(*args, **kwargs)
        self.help_text = "Images over 500kb will be resized to keep under 500kb limit, which may result in some loss of quality"

    def validate(self,image):
        if not str(image).split('.')[-1].lower() in ["jpg","jpeg","png","gif"]:
            raise ValidationError("File format not supported, please try again and upload a JPG/PNG/GIF file")

    def to_python(self, image):
        try:
            limit = 500000
            num_of_tries = 10
            img = Image.open(image.file)
            width, height = img.size
            ratio = float(width) / float(height)

            upload_dir = settings.FILE_UPLOAD_TEMP_DIR if settings.FILE_UPLOAD_TEMP_DIR else '/tmp'
            tmp_file = open(os.path.join(upload_dir, str(uuid.uuid1())), "w")
            tmp_file.write(image.file.read())
            tmp_file.close()

            while os.path.getsize(tmp_file.name) > limit:
                num_of_tries -= 1
                width = 900 if num_of_tries == 0 else width - 100
                height = int(width / ratio)
                img.thumbnail((width, height), Image.ANTIALIAS)
                img.save(tmp_file.name, img.format)
                image.file = open(tmp_file.name)
                if num_of_tries == 0:
                    break                
        except:
            pass
        return image