feat(media): support arbitrary file types with download cards (#810)

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Brain <21994759fc7a6fa6b965551d35cfd7897d262f2495467f2d78694ddcfa6a5c7e@sprout-oss.stage.blox.sqprod.co>

desktop/src/shared/ui/markdownFileCard.ts

@@ -0,0 +1,42 @@
+import { rewriteRelayUrl } from "@/shared/lib/mediaUrl";
+
+/** Minimal shape of an imeta entry as consumed by the markdown renderer. */
+export type FileCardImetaEntry = {
+  m?: string;
+  size?: number;
+  filename?: string;
+};
+
+export type ResolvedFileCard = {
+  href: string;
+  filename: string;
+  size?: number;
+};
+
+/**
+ * Decide whether a markdown link should render as a generic-file download
+ * card. A link qualifies when its href matches an imeta entry whose MIME is
+ * neither image nor video (media goes through the `img` renderer instead).
+ *
+ * Pure — extracted from `markdown.tsx` so the FileCard decision (the riskiest
+ * part of the generic-file rendering path) is unit-testable without mounting
+ * React. Returns the resolved card props, or `null` to fall through to normal
+ * link handling.
+ */
+export function resolveFileCard(
+  entry: FileCardImetaEntry | undefined,
+  href: string | undefined,
+  childText: string,
+): ResolvedFileCard | null {
+  if (
+    !href ||
+    !entry?.m ||
+    entry.m.startsWith("image/") ||
+    entry.m.startsWith("video/")
+  ) {
+    return null;
+  }
+  const filename =
+    entry.filename || childText.trim() || href.split("/").pop() || "file";
+  return { href: rewriteRelayUrl(href), filename, size: entry.size };
+}