- IsReadmeFile now takes (name, mode) and only matches regular file blobs (filemode.Regular/Executable). A directory or symlink named "readme" was being picked up by tree handlers, which then tried to fetch its blob and 503'd. - ReadmePattern matches by convention (^readme(?:[._-].+)?$) rather than a fixed extension set. README.rst, README.org, README-old, etc. now surface on index/tree pages and fall through to FormatText in GetFormat for plaintext rendering. - pages.RepoIndex and pages.RepoTree route through markup.GetFormat instead of hardcoded extension switches, so the markdown extension list lives in one place (FileTypePatterns[FormatMarkdown]). - Added format_test.go covering IsReadmeFile (mode/symlink/dir rejection, the directory regression), FileTypePatterns, and GetFormat. Signed-off-by: Evan Jarrett <evan@evanjarrett.com>