promote links to file assets unless we already know they are pages

[Fil]

closes #639

Note: in #639 Mike mentions “remove .md”, which I take as a suggestion that we might want a link to [page.md](page.md) to be considered as a link to the page.

With this PR I don't apply that change, and instead a link to [page.md](page.md) results in a file asset with the source markdown. If that is not what we want, then we could add .md to the skip list.

I also wonder if we need to treat a[href][download] as always linking to a file?