fix(media): also allow host-local Markdown uploads

Extend the text/plain sniff exception to cover text/markdown in addition
to text/csv. Both formats are structurally indistinguishable from plain
text at the byte level, so the same pattern applies.

src/media/web-media.test.ts

@@ -198,6 +198,19 @@ describe("loadWebMedia", () => {
     expect(result.contentType).toBe("text/csv");
   });
 
+  it("allows host-read Markdown files", async () => {
+    const mdFile = path.join(fixtureRoot, "notes.md");
+    await fs.writeFile(mdFile, "# Title\n\nSome **bold** text.\n", "utf8");
+    const result = await loadWebMedia(mdFile, {
+      maxBytes: 1024 * 1024,
+      localRoots: "any",
+      readFile: async (filePath) => await fs.readFile(filePath),
+      hostReadCapability: true,
+    });
+    expect(result.kind).toBe("document");
+    expect(result.contentType).toBe("text/markdown");
+  });
+
   it("rejects binary data disguised as a CSV file", async () => {
     const fakeCsv = path.join(fixtureRoot, "evil.csv");
     // Write a PNG header — binary, not text

src/media/web-media.ts

@@ -84,6 +84,7 @@ const HOST_READ_ALLOWED_DOCUMENT_MIMES = new Set([
   "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
   "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
   "text/csv",
+  "text/markdown",
 ]);
 const MB = 1024 * 1024;
 
@@ -134,11 +135,13 @@ function assertHostReadMediaAllowed(params: {
     return;
   }
   const normalizedMime = normalizeMimeType(params.contentType);
-  // CSV exception: content sniffers report text/plain for CSV because CSV is structurally
-  // indistinguishable from plain text at the byte level. Allow it when:
-  // - The extension-derived MIME is text/csv (operator intent)
+  // CSV and Markdown exception: content sniffers report text/plain for both formats
+  // because they are structurally indistinguishable from plain text at the byte level.
+  // Allow them when:
+  // - The extension-derived MIME is text/csv or text/markdown (operator intent)
   // - The content sniffed as text/plain (confirming valid text, not binary data)
-  if (sniffedMime === "text/plain" && normalizedMime === "text/csv") {
+  const TEXT_PLAIN_ALIASES = new Set(["text/csv", "text/markdown"]);
+  if (sniffedMime === "text/plain" && normalizedMime && TEXT_PLAIN_ALIASES.has(normalizedMime)) {
     return;
   }
   if (