[spec] adding a "magic number" for gemini files

[cage]

On Sun, Dec 27, 2020 at 05:18:08PM -0500, Sean Conner wrote:

[...]

>
>   In the configuration file, the user can specify a mapping of extensions to
> MIME type, to which the default extension type that identifies a Gemini
> index file is added (which for my server is ".gemini" but that can be
> changed).
>
>   During a request, the extension is checked against this mapping.  If (for
> a non-Gemini file) this doesn't exist, only then is libmagic queried.
>
>   -spc

Crystal clear, thank you! :)
C.