path_or_file argument of flask.helpers.send_file is typed as os.PathLike[t.AnyStr] | str | t.BinaryIO. This prevents passing some objects that are t.IO[bytes], but not t.BinaryIO. The underlyingwerkzeug.utils.send_file already allows t.IO[bytes] due to pallets/werkzeug#2209 since version 2.0.2.

Reproduction:

from tempfile import NamedTemporaryFile
from typing import IO

from flask.helpers import send_file


# The return type cannot be "BinaryIO" because "NamedTemporaryFile" is incompatible with it according to Mypy.
def some_function() -> IO[bytes]:
    file = NamedTemporaryFile()
    ...
    return file


file = some_function()
send_file(file)

Raises the following exception with Mypy 1.16.1.

error: Argument 1 to "send_file" has incompatible type "IO[bytes]"; expected "PathLike[str] | str | BinaryIO"  [arg-type]

I could simply change the return value of some_function to _TemporaryFileWrapper[bytes] or cast it to BinaryIO.
However, I would like to allow t.IO[bytes].

Side note: _TemporaryFileWrapper[bytes] conforms to PathLike[str] due to python/typeshed#7840. That is why it is accepted by flask.helpers.send_file.

Allowing t.IO[bytes] would be backwards compatible change as all t.BinaryIO are also t.IO[bytes].
Mypy Playground

Environment:

• Python version: 3.12.11
• Flask version: 3.1.1