Flask app crashes when using send_file() with BytesIO object in debug mode

[kingpig0117]

When using send_file() with a BytesIO object while debug=True, the application raises a ValueError on the second request to the same endpoint.

Environment:

• Python version: 3.11.4
• Flask version: 3.0.2
• Werkzeug version: 3.0.1
• OS: Ubuntu 22.04

Steps to Reproduce:

from flask import Flask, send_file
from io import BytesIO

app = Flask(__name__)

@app.route('/download')
def download():
    buffer = BytesIO()
    buffer.write(b'Hello World')
    buffer.seek(0)
    return send_file(buffer, mimetype='text/plain', download_name='hello.txt')

if __name__ == '__main__':
    app.run(debug=True)

1. Run the app with python app.py
2. Visit http://127.0.0.1:5000/download once (works fine)
3. Refresh the page (crashes)

Expected Behavior:

The file should download successfully on every request.

Actual Behavior:

ValueError: I/O operation on closed file.

[davidism]

I can't reproduce this issue with the example provided. Multiple requests result in successful responses. There's something else going on beyond what you've shown.