Instead of relying solely on a mime.types file, pine 4.10 (in
./pine/send.c, around line 6910) looks at the first few bytes of an
attachment to determine its type.  I've had several MS Excel 97 files which
pine 4.10 slaps an "APPLICATION/MSWORD" mime type onto, which can mess up
the recipient's ability to view the attachment directly from his/her MUA
(in two cases of mine the recipients were using Netscape 4.61/win95 and MS
Outlook/WinNT).

I don't know what the "correct" fix is, but in my case changing the subtype
from "MSWORD" to "octet-stream" took care of it.  I've sent email to
pine-info.edu but haven't received anything back.