from telethon import TelegramClient
from config import config
client = TelegramClient(config.SESSION_NAME, config.TG_API_ID, config.TG_API_HASH)

async def main(img_path=None):
    async for dialog in client.iter_dialogs():
        if dialog.is_channel:
            if dialog.id == -123456789:
                print(f'{dialog.id}: {dialog.title}')
                channel_from = dialog
    messages = await client.get_messages(channel_from, limit=None)
    for message in messages:
        if message.file:
            path = await client.download_media(message.media, "/tmp/output")
            print(f'File saved to {path}')  # printed after download is done