C-c C-x RET insert a mail message.
C-c C-x C-s insert a signature file at end.
C-c C-x t insert a new MIME tag.
-C-c C-x a enclose as multipart/alternative.
-C-c C-x p enclose as multipart/parallel.
-C-c C-x m enclose as multipart/mixed.
-C-c C-x d enclose as multipart/digest.
-C-c C-x s enclose as PGP signed.
-C-c C-x e enclose as PGP encrypted.
+C-c C-m C-a enclose as multipart/alternative.
+C-c C-m C-p enclose as multipart/parallel.
+C-c C-m C-m enclose as multipart/mixed.
+C-c C-m C-d enclose as multipart/digest.
+C-c C-m C-s enclose as PGP signed.
+C-c C-m C-e enclose as PGP encrypted.
C-c C-x C-k insert PGP public key.
-C-c C-x C-p preview editing MIME message.
+C-c C-x p preview editing MIME message.
...
therefore, you should type C-c C-x C-i and specify the binary file
@item @key{C-c C-m C-a}
Enclose specified region as multipart/alternative.
-@item @key{C-c C-x C-p}
+@item @key{C-c C-m C-p}
Enclose specified region as multipart/parallel.
-@item @key{C-c C-x C-m}
+@item @key{C-c C-m C-m}
Enclose specified region as multipart/mixed.
-@item @key{C-c C-x C-d}
+@item @key{C-c C-m C-d}
Enclose specified region as multipart/digest.
-@item @key{C-c C-x C-s}
+@item @key{C-c C-m C-s}
Digital-sign to specified region. (cf. @ref{PGP})
-@item @key{C-c C-x C-e}
+@item @key{C-c C-m C-e}
Encrypt to specified region. (cf. @ref{PGP})
-@item @key{C-c C-x C-q}
+@item @key{C-c C-m C-q}
avoid to encode tags in specified region. In other words, tags is
interpreted as such string. (In current version, it may be
incomplete. Maybe PGP-signature does not work for this enclosure.)