`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

Start by comparing the email.mime.image.MIMEImage and email.message.MIMEPart implementations, focusing on how each uses the supplied EmailPolicy during base64 encoding. Reproduce the example with max_line_length=35 and confirm that MIMEImage emits lines no longer than the policy allows; linked PR gh-133322 indicates work is already underway.