django signals, how to use "instance"

I am trying to create a system which enables user to upload a zipfile, and then extract it using post_save signal.

class Project:
    ....
    file_zip=FileField(upload_to='projects/%Y/%m/%d')

@receiver(post_save, sender=Project)
def unzip_and_process(sender, **kwargs):
    #project_zip = FieldFile.open(file_zip, mode='rb')
    file_path = sender.instance.file_zip.path
    with zipfile.ZipFile(file_path, 'r') as project_zip:
        project_zip.extractall(re.search('[^\s]+(?=\.zip)', file_path).group(0))
        project_zip.close()

unzip_and_process method works fine when correct file paths are provided(in this case, i need to provide instance.file_zip.path. However, I couldn't get/set the instance with the signals. Django documentation about signals is not clear and have no examples. So, what do I do?