Question for Davefes : jpeg file attached to email

[PatLub85]

Hi,
I would like to send a jpg file (photo taken by an ESP32-Cam) as an attachment to an email.
I saw the discussion #16844, and the Davefes website https://github.com/davefes/My-uMail-and-examples/tree/main and I have a question on this subject:
What is the boundary() part for?

[davefes]

I have no idea, just used the code at:
shawwwn/uMail#2

Seems to be associated with:

if attachment:
   text_id = boundary()
   attachment_id = boundary()

MIME boundaries
https://www.oreilly.com/library/view/programming-internet-email/9780596802585/ch03s04.html

[PatLub85]

I am not an expert in email protocol.
After a little research on the web, I understand the usage, but why use a random separation?
https://fr.wikipedia.org/wiki/Multipurpose_Internet_Mail_Extensions

[Josverl]

On protocol level the message and attachments are send as a single stream of text characters.

This used to simple, The S in SMTP,
But got more complext with the addition of formatting and attachments.

The boundary is a demarcation between parts of the message, usually between attachments.
But if we would agree on just one boundary/ demarcation as a string of some characters, then you could never attach a document contains that demarcation.
So a mail client must generate a boundary string that is guaranteed does not occur in the base64 encoded attachment to that mail message.

And that allows to send multi-part MIME messages.

https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html