bug#51523: 29.0.50; gnus-mime-view-part-externally very slow

[Stefan Monnier]

> And that is indeed very slow.  But I don't think any code is doing that?
> The code is doing (mailcap-mime-info "application/octet-stream").

The profile gave a pretty good hint:

        9863  80%    - gnus-mime-view-part-externally
        9787  80%     - gnus-mime-view-part-as-type
        9374  76%      - seq-filter
        9374  76%       - seq-map
        9374  76%        - apply
        9374  76%         - #<compiled -0x1c9911d9>
        9374  76%          - mapcar
        9374  76%           - #<compiled 0xd7171c>
        9370  76%            - #<compiled 0x1d5f69f8>
        9330  76%             - mailcap-mime-info

So, yes, we do have code that performs such a loop, in
`gnus-mime-view-part-as-type`.


        Stefan