you can try it.

def encode_filename(filename:str) -> str:
    encoded_filename = base64.b64encode(filename.encode('utf-8')).decode('utf-8')
    return f'=?utf-8?B?{encoded_filename}?='

file_name = "Avent à à écouter.pdf"
print(encode_filename(file_name))    # =?utf-8?B?QXZlbnQgw6Agw6Agw6ljb3V0ZXIucGRm?=