>>>>> In <v9ack9lkaa.fsf@marauder.physik.uni-ulm.de> Reiner Steib wrote:

> the doc string of `mml-attach-file' says that TYPE is optional:

And also for `mml-attach-external'.

[...]

> After evaluating (mml-attach-file "/tmp/ste/gnus/test-file-1.txt") in
> a message buffer, I get:

>| <# part filename="/tmp/ste/gnus/test-file-1.txt" disposition=attachment>
>| <# /part>

> Note: type="text/plain" is not specified.

It is the same in the case where `mml-attach-external' is used.

> When sending the message, I get the backtrace listed below[1].

Aren't you talking about `mml-attach-external'?  Though I couldn't
reproduce the error because the type defaults to text/plain for the <#
part ...>s internally in `mml-generate-mime-1', I think it isn't good
that Gnus doesn't add the Content-Type MIME header to the part anyway,
and the default type should be application/octet-stream if it cannot
be guessed.

> I'm not sure about the best way to fix this problem:

> (1) Make TYPE in `mml-attach-file' mandatory.

> (2) Set Content-Type in `mml-generate-mime-1' appropriately if no
>     type=".../..." is specified.

> Patch for solution (2):

[...]

> Opinions?  Unless there are other suggestion, I'll install (2).

I vote to (2).  Here're patches for the v5-10 branch and for the
trunk, which enable to add the Content-Type header for the <#
part ...>s as well.