Separate .mmd or .mermaid support

[deadcoder0904]

If someone has a .mmd or .mermaid, this extension doesn't work there even though it works flawlessly if embedded in .md file.

Is there a chance to have that?

[soreavis]

This looks doable, and it can reuse the existing mermaid path. Mermaid already renders inside .md because the pipeline turns a ```mermaid fenced block into a <div class="mermaid"> and injects the script — so a standalone file could go through the same path:

1. Register the type — add the .mmd/.mermaid UTIs to the Quick Look extension's QLSupportedContentTypes (the same mechanism used for the other Markdown variants), so Finder routes those files to QLMarkdown.
2. Wrap on render — when the previewed file has a mermaid extension, wrap its contents in a ```mermaid fence before parsing. The existing transform then renders it; I confirmed locally that feeding a raw diagram through a fence produces the <div class="mermaid"> + script, so no new rendering code is needed.

The one thing worth deciding first is .mmd, which is ambiguous: it's also MultiMarkdown's extension (and Mermaid's own CLI uses it). .mermaid is unambiguous and safe to always treat as a diagram. For .mmd there are a few options:

• treat it as a Markdown document (embedded mermaid still works, standalone doesn't),
• always treat it as a diagram (breaks MultiMarkdown .mmd files),
• a small content sniff (first non-empty line looks like graph / flowchart / sequenceDiagram / … → diagram, otherwise Markdown),
• or a preference toggle.

@sbarex, if you have a preference on the .mmd behaviour — or want to start with just .mermaid — I'm happy to put together a PR. The registration plus the wrap step is small and self-contained.

[sbarex]

Implemented only for .mermaid files on release 1.5.2.