Notes/Domino 6 and 7 Forum : "Missing" attachment in multipart/related MIME email [longish

Here's a problem I have not seen before, but wonder if others have?

A supplier of mine emailed me the other day with words to the effect of "see attached file" - and there was no attached file. That is, the message body, still in native MIME, had no attachment icon visible to me, so no way to detach any attachment. Also, the same message in the Inbox view had no attachment icon.

Naturally I suspected the sender (nitwit forgot to attach the file...), so I emailed back and asked for it again.

Got exactly the same thing back, but this time noticed that the size of the message was quite large - much larger, in fact, than would have been necessary just to say to me "see attached file", plus an over elaborate signature block, minus the required attachment, even if the whole thing was in HTML. So I looked at the message source and found this:

[headers snipped]

Content-Type: multipart/related; boundary="----_=_NextPart_000_01C2840E.2277A470"; type="multipart/alternative"

------_=_NextPart_000_01C2840E.2277A470 Content-Type: multipart/alternative; boundary="----_=_NextPart_001_01C2840E.2277A470" ------_=_NextPart_001_01C2840E.2277A470 Content-Type: text/plain

[plain text version of message snipped]

------_=_NextPart_001_01C2840E.2277A470 Content-Transfer-Encoding: quoted-printable Content-Type: text/html

[text/html version of message snipped]

------_=_NextPart_001_01C2840E.2277A470-- ------_=_NextPart_000_01C2840E.2277A470 Content-Type: application/octet-stream; name="VanDirect4ExtServer.zip" Content-Disposition: attachment; filename="VanDirect4ExtServer.zip" Content-Transfer-Encoding: base64
[enormous base64 encoded attachment snipped]

------_=_NextPart_000_01C2840E.2277A470 Content-Type: image/gif; name="image001.gif" Content-Disposition: attachment; filename="image001.gif" Content-ID: <image001.gif@01C2840E.04E39EF0> Content-Transfer-Encoding: base64

[supplier's logo - which did render correctly in the HTML representation of the email - snipped]

------_=_NextPart_000_01C2840E.2277A470--

[end]

So I selected the content of the part containing the attachment and copied/pasted/saved it as a text file, then ran my pet base64 decoder over it and found... The Missing Attachment.

It was there all along, but the Notes client didn't recognise it.

Question: What is broken, Notes' ability to interpret multipart/MIME messages or the MIME in this message (from an Exchange server as far as I can tell)?

Any ideas?