send_file not inserting quote around filename

[Timmy93]

Overview

The function send_file does not insert quotes around the filename in the Content-Disposition header when the sent file does not have spaces. (See here for MDN docs)
This causes browser (tested on Windows 11 - last version to date of Firefox and Chrome) to download the file with a blank name (respectively as undefined and undefined.txt)

To recap:

• when test.pdf is sent with send_file the following Content-Disposition header is set to: attachment; filename=test.pdf -> Correct file is sent but with wrong filename
• when test 1.pdf is sent with send_file the following Content-Disposition header is set to: attachment; filename="test 1.pdf" -> Works perfectly

Bug replication

Send a file without spaces in name:

from flask import Flask, send_file
app = Flask(__name__)

filepath = "test.pdf"
  if filepath:
    response = send_file(filepath, as_attachment=True)

Expected behavior:

• when test.pdf is sent with send_file the following Content-Disposition header is set to: attachment; filename="test.pdf"
• when test 1.pdf is sent with send_file the following Content-Disposition header is set to: attachment; filename="test 1.pdf"

Environment:

• Python version: 3.11
• Flask version: 3.0.0
• Waitress: 2.1.2

[Sympatron]

The problem comes from these lines in quote_header_value:

werkzeug/src/werkzeug/http.py

Lines 165 to 169 in d3dd65a

	if allow_token:
	token_chars = _token_chars
	if token_chars.issuperset(value):
	return value

This is used called from dump_options_header.

Do you know if this is true for all headers? If yes, it may be safer to default to allow_token = False.
If not dump_options_header, _options_header_vkw and Headers.set should accept allow_token and pass it on.

[Timmy93]

Honestly I don't know, manually adding the quotes using a regex fixed the problem, but I imagine it is not the correct solution, it's just a workaround.

[davidism]

I can't reproduce this issue. Sending a filename without spaces is downloaded with the correct filename by Firefox, Vivaldi (Chrome), and Safari. The Content-Disposition filename parameter can be either unquoted token chars, or a quoted string if needed, https://httpwg.org/specs/rfc6266.html#header.field.definition, both are valid.

[Timmy93]

The problem can be reproduced if you set your language to italian and you try firefox to download any file like "test.pdf"