A `.md` delivered via MEDIA has no entry in MEDIA_BY_EXT, so mediaKind()
classified it as a generic 'file' and MarkdownLink rendered a download-style
anchor. Markdown is renderable content: route markdown document paths to
PreviewAttachment (source='tool-result'), which opens them in the right-rail
preview pane — where .md already renders with a rendered/source toggle and,
since #89381, full KaTeX math, tables, images, and links.
Resolves#84951 (the MEDIA delivery half; the rail-side rendering half landed
in #89381).
Teknium
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