Bug report

Bug description:

if variable 'attachment' is of type email.message.Message

attachment.get_payload(decode=True) 

Will yield the base64 content rather than the unecoded content if the sender provides the header 'base64 '

IE

attachment['Content-Transfer-Encoding']
'base64 '  <-notice the extra space

If we adjusted this code
https://github.com/python/cpython/blob/3.12/Lib/email/message.py#L290
to also strip(), this would resolve those edge cases where email clients erroneously add spaces.

This was an actual problem I have run into with a vendor.

CPython versions tested on:

3.9, 3.12

Operating systems tested on:

macOS

Linked PRs

• gh-123742: Fixed get_payload not being able to parse headers with spaces #123761