Backport #31825 by @Zettat123
Fix#31395
This regression is introduced by #30273. To find out how GitHub handles
this case, I did [some
tests](https://github.com/go-gitea/gitea/issues/31395#issuecomment-2278929115).
I use redirect in this PR instead of checking if the corresponding `.md`
file exists when rendering the link because GitHub also uses redirect.
With this PR, there is no need to resolve the raw wiki link when
rendering a wiki page. If a wiki link points to a raw file, access will
be redirected to the raw link.
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Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: yp05327 <576951401@qq.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
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