fix(eve): prefer known Telegram attachment media type over HTTP content-type

.changeset/telegram-attachment-media-type.md

@@ -0,0 +1,5 @@
+---
+"eve": patch
+---
+
+Telegram channel: prefer the known attachment media type over the HTTP `content-type` header when fetching files. Telegram's file endpoint frequently returns `application/octet-stream` (or no content-type) for photos, which broke image recognition in vision models that key off the declared media type.

packages/eve/src/public/channels/telegram/attachments.test.ts

@@ -86,4 +86,39 @@ describe("createTelegramFetchFile", () => {
 
     await expect(fetchFile("https://example.com/file.png")).resolves.toBeNull();
   });
+
+  it("prefers the known attachment media type over the HTTP content-type header", async () => {
+    // Telegram's file endpoint frequently returns `application/octet-stream`
+    // (or no content-type) for photos. The channel already knows the photo is
+    // `image/jpeg` from the inbound message; that known type must win so vision
+    // models receive a recognized image media type.
+    const fetchMock = vi
+      .fn()
+      .mockResolvedValueOnce(
+        new Response(JSON.stringify({ ok: true, result: { file_path: "photos/file.jpg" } }), {
+          headers: { "content-type": "application/json" },
+        }),
+      )
+      .mockResolvedValueOnce(
+        new Response("JFIF", { headers: { "content-type": "application/octet-stream" } }),
+      );
+
+    const fetchFile = createTelegramFetchFile({
+      api: { apiBaseUrl: "https://telegram.example", fetch: fetchMock },
+      credentials: { botToken: "123456:ABCDEF" },
+      policy: { allowedMediaTypes: ["image/*"], maxBytes: 1024 },
+    });
+
+    const result = await fetchFile(
+      String(
+        createTelegramFileUrl({
+          fileId: "photo-id",
+          filename: "photo.jpg",
+          mediaType: "image/jpeg",
+        }),
+      ),
+    );
+
+    expect(result?.mediaType).toBe("image/jpeg");
+  });
 });

packages/eve/src/public/channels/telegram/attachments.ts

@@ -90,8 +90,13 @@ export function createTelegramFetchFile(input: {
     }
 
     const bytes = Buffer.from(await response.arrayBuffer());
+    // Prefer the media type the channel already knows (from the attachment
+    // metadata, e.g. `image/jpeg` for photos) over the HTTP `content-type`
+    // header returned by Telegram's file endpoint, which is frequently
+    // `application/octet-stream` or unset. Using the latter breaks image
+    // recognition in vision models that key off the declared media type.
     const mediaType =
-      response.headers.get("content-type") ?? ref.mediaType ?? "application/octet-stream";
+      ref.mediaType ?? response.headers.get("content-type") ?? "application/octet-stream";
     const result: FetchFileResult = {
       bytes,
       filename: ref.filename,