N-K-O wrote:
As I see it, a MIME message can contain the following internet properties in its header:
At least you are looking into part of the actual International Internet Standards that govern email creation and transfer.
Content-Type: multipart/mixed:
Multipart/mixed is used for sending files with different Content-Type header fields inline (or as attachments) (from https://en.wikipedia.org/wiki/MIME)
All those «Content-Type» headers are for attachments, regardless how such may be formatted into the Body of the email.
This, of course, is not truly appropriate if all «Content-Type[s]» are the same, but no one will “arrest” you, if you use it for such cases.
Content-Type: multipart/related:
A multipart/related is used to indicate that each message part is a component of an aggregate whole (from https://en.wikipedia.org/wiki/MIME), i.e. embedded images.
Again, of course, all the parts (whether “related” or “mixed”) are attachments, regardless how they may be formatted within the email Body.
What the good folks here are complaining about is that Apple Mail at one time used the Content-Type: multipart/mixed header, and at some point switched to specifying Content-Type: multipart/related if the only content were JPGs.
The mail tool I use (as well as Apple Mail form IOS 12, I confirm after a test this morning) sends a JPG with Content-Type: multipart/mixed.
That is an interesting find, and may explain why certain “workarounds” succeeded at getting the very tiny subset of email clients (consisting of a single email client and its derivatives, according to all testing I, and others, have seen or accomplished) to “do the right thing”™️.
This may suggest that the problem, with that very tiny subset of email clients, is that they will only allow their users to perform bulk attachment operations (such as bulk-saves) when the MIME header, for the attachments, is «multipart/mixed».
Why would that be?
What “logic” makes that reasonable?
I could, almost, see a sort of “logic” in the opposite behavior: at least the bulk operation is being applied to “related” parts.
Once again, we find a lack of standards compliance, causing all the hassles y’all have been experiencing with that very tiny subset of email clients.
No other email clients have this—shall I say, rather “odd”—misbehavior, with email attachments.
To me this is not a matter of compliance, both are compliant MIME messages, it was a choice of which header to include. And as we have discovered, selecting a video and emailing it with Apple Mail will result in Content-Type: multipart/mixed, meaning the tool can include either header, but chooses not too for JPG attachments.
As I said, we do, perhaps, see why that “workaround” actually succeeded at getting that very tiny subset of email clients to “do the right thing”™️, even if the “reason” is quite “odd”.
Unfortunately, there is nothing in any of the International Internet Standards that require any email client to ever provide bulk attachment operations (such as bulk saves) to their users.
So. Sure. That very tiny subset of email clients is not—technically—out of compliance with any such standard.
However. Nonetheless, it is strictly that very tiny subset of email clients that has this misbehavior.
One should always be allowed to correctly designate when all email attachments are of the same type. Such may even provide various efficiencies in the transfer and other handling of such emails.
This is my understanding, but am no export at all on MIME format.
Thank you for this contribution.
As I’ve already written, this does seem to provide a clue to a certain set of “workarounds”.
Unfortunately, it provides no clue to the “reasoning” behind the misbehavior of that very tiny subset of email clients.
However, it may provide a good “target” in trying to get the creator of that very tiny subset of email clients to correct this misbehavior.