AskFileMessage

import chainlit as cl


@cl.on_chat_start
async def start():
    files = None

    # Wait for the user to upload a file
    while files == None:
        files = await cl.AskFileMessage(
            content="Please upload a text file to begin!", accept=["text/plain"]
        ).send()

    text_file = files[0]

    with open(text_file.path, "r", encoding="utf-8") as f:
        text = f.read()

    # Let the user know that the system is ready
    await cl.Message(
        content=f"`{text_file.name}` uploaded, it contains {len(text)} characters!"
    ).send()

import chainlit as cl

file = await cl.AskFileMessage(
        content="Please upload a python file to begin!", accept={"text/plain": [".py"]}
      ).send()