folded file names in MIME headers are interpreted incorrectly by part.get_filename()

If the filename in the header field of a MIME attachment is folded, the `get_filename()` function doesn't return the correct file name. It's supposed to ignore folded CRLFs but doesn't.

Script which illustrates the bug:

#!/usr/bin/env python3

import email

message = email.message_from_string("""\
MIME-Version: 1.0
Content-Type: text/plain; name="This File Name Is
 Folded.pdf"
Content-Disposition: attachment; filename="This File Name Is
 Folded.pdf"
Content-Transfer-Encoding: 8bit

Foo
""")

print(message.get_filename())

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: libpython3.10-minimal 3.10.4-3
ProcVersionSignature: Ubuntu 5.15.0-39.42-generic 5.15.35
Uname: Linux 5.15.0-39-generic x86_64
ApportVersion: 2.20.11-0ubuntu82.1
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: ubuntu:GNOME
Date: Sun Jun 19 15:49:23 2022
InstallationDate: Installed on 2019-01-02 (1264 days ago)
InstallationMedia: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.3)
SourcePackage: python3.10
UpgradeStatus: Upgraded to jammy on 2022-02-20 (118 days ago)