gemserv: Non-gemini files have mime type text/gemini

[tastytea]

For example, <gemini://tastytea.de/dotfiles/.emacs.d/init.el> returns
“20 text/gemini”¹. Files without a file extension are also returned as
text/gemini.

I think text/plain is better suited as the default type for unknown
files.

¹ echo -en 'gemini://tastytea.de/dotfiles/.emacs.d/init.el\r\n' \
      | openssl s_client -connect tastytea.de:1965 -quiet | head -n 1
-- 
Get my PGP key with `gpg --locate-keys tastytea at tastytea.de` or at
<https://tastytea.de/tastytea.asc>.
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