Python build multipart

# модуль mimetypes не может работать с кириллическими типами(например, "аудио/AMR") и падает
mime_types = {
    'bmp': 'image/bmp',
    'gif': 'image/gif',
    'jpg': 'image/jpeg',
    'jpeg': 'image/jpeg',
    'png': 'image/png'
}

def build_multipart(boundary, fields, files=None):
    L = ['']
    for k, v in fields.items():
        L.append('Content-Disposition: form-data; name="' + k + '"\r\n\r\n' + v)
    if files:
        for k, v in files.items():
            f = open(v, 'r')
            data = f.read()
            f.close()
            filename = v.replace('\\', '/').split('/')[-1]
            parts = filename.split('.')
            extension = parts[-1].lower() if len(parts) > 1 else ''
            content_type = mime_types[extension] if extension in mime_types else 'application/octet-stream'
            L.append('Content-Disposition: form-data; name="' + k + '"; filename="' + filename + '"\r\nContent-Type: ' + content_type + '\r\n\r\n' + data)
    return ('\r\n--' + boundary + '\r\n').join(L) + '\r\n--' + boundary + '--\r\n'