use IO[bytes] instead of BinaryIO for wider compatibility (#5777)

CHANGES.rst

@@ -5,6 +5,7 @@ Unreleased
 
 -   When using ``follow_redirects`` in the test client, the final state
     of ``session`` is correct. :issue:`5786`
+-   Relax type hint for passing bytes IO to ``send_file``. :issue:`5776`
 
 
 Version 3.1.1

src/flask/helpers.py

@@ -398,7 +398,7 @@ def _prepare_send_file_kwargs(**kwargs: t.Any) -> dict[str, t.Any]:
 
 
 def send_file(
-    path_or_file: os.PathLike[t.AnyStr] | str | t.BinaryIO,
+    path_or_file: os.PathLike[t.AnyStr] | str | t.IO[bytes],
     mimetype: str | None = None,
     as_attachment: bool = False,
     download_name: str | None = None,