Attachment to received email not shown at all in Mail

Question

I am using Mail and Safari on my Mac Mini with OS 10.8.5 (Mountain Lion). My email provider and ISP is BT/Yahoo!
I received an email that had been written on and sent from an iPad. It was supposed to contain a one-page MS Word (.docx) text file as an attachment. When I opened the email in Mail, there was no attachment, and I was on the point of complaining to the sender that she had failed to attach the file, when I had the idea of looking at the email in my BT/Yahoo! webmail — and the attachment was there.
The only clue I have as to why the attachment did not show up when the email was opened in Mail is that the message in the email was prefaced by the following:
------=_Part_841708_1134486248.1416002644218
Content-Type: multipart/alternative;
  boundary="----=_Part_841707_1414804683.1416002644201"
------=_Part_841707_1414804683.1416002644201
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: quoted-printable
Can this be the attachment in disguise? At any rate I should be very grateful for any explanations and, in particular, for advice about making sure that attachments do appear in Mail in the future.
I should add that I have not experienced this phenomenon before — though the non-appearance of an attachment might not have been noticed if I did not know that it should have been there.

Answer 1

I realize that I should not see headers like that, and none of my other received emails that day had them. The particular email in question had other unwanted characters within its message, suggesting that something gone wrong with its encoding. Here is a small sample (i.e. the '=20'):
Subject: Re: Draft email to Claire re topics /possible agenda items for Wa=
ndle consultative group mtg
=20
The email was part of a conversation, and earlier parts of it had far more of these intrusive characters, which were not there when those parts of the conversation were first received. Here is a sample:
I hope you still have your full revised text and can send it to us again.
Best,Michael.=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0=
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0Michael Graubart
I did not, in fact, do View / Message / Raw Source when I read the email. I have now gone back to the email and have now tried View / Message / Raw Source to see what the result would be. What happened was that an additional view of the message opened alongside the first one, with an enormous mass of characters which I shall not reproduce here unless I am told that it would be helpful.
But in neither of the views could I find a trace of the attachment. That is what really worries me, in case it happens again unknown to me.