`email.mime.image.MIMEImage`'s `policy` argument doesn't take effect

Bug report

Bug description:

I'm comparing the creation of two MIME parts, one using MIMEImage and one using MIMEPart, with the same policy object, which asks for max_line_length=35.

the MIMEImage does not respect the max_line_length when base64-encoding, but the MIMEPart does:

#!/usr/bin/python3

from io import BytesIO
from email.policy import EmailPolicy
from email.mime.image import MIMEImage
from email.message import MIMEPart
from PIL import Image, ImageDraw

img = Image.new('RGBA', (20,20))
d = ImageDraw.Draw(img)
d.regular_polygon((10,6,6), 3, fill='red')
d.line([(16,10), (10,16)], fill='blue', width=2)
d.line([(16,16), (10,10)], fill='blue', width=2)
b = BytesIO()
img.save(b, format='png', optimize=True)
imgbytes = b.getvalue()

policy = EmailPolicy(max_line_length=35)

f = MIMEImage(imgbytes, policy=policy)
print(f)

g = MIMEPart(policy=policy)
g.set_content(imgbytes, 'image', 'png')
print(g)

The output is:

Content-Type: image/png
MIME-Version: 1.0
Content-Transfer-Encoding: base64

iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbAMAgS739nO3Tp
Rw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZsgrzfcqVMpL2jo0447gYDpeA
rk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/ulivdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

Content-Type: image/png
Content-Transfer-Encoding: base64

iVBORw0KGgoAAAANSUhEUgAAABQAAAAU
CAYAAACNiR0NAAAAcElEQVR42uVTOxbA
MAgS739nO3TpRw20dqpbfARQEjOywiwY
nCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
sgrzfcqVMpL2jo0447gYDpeArk+OnJHk
IhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJg
gg==

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-133322

まず、email.mime.image.MIMEImage と email.message.MIMEPart の実装を比較し、base64 エンコード時にそれぞれが指定された EmailPolicy をどのように使用するかに注目してください。max_line_length=35 で例を再現し、MIMEImage が policy で許可される長さを超えない行を出力することを確認してください。リンクされた PR gh-133322 は、作業がすでに進行中であることを示しています。