diff --git a/packages/host-service/src/trpc/router/attachments/attachments.test.ts b/packages/host-service/src/trpc/router/attachments/attachments.test.ts
index 5dc6ac0cec1..8b606ac8a10 100644
--- a/packages/host-service/src/trpc/router/attachments/attachments.test.ts
+++ b/packages/host-service/src/trpc/router/attachments/attachments.test.ts
@@ -84,14 +84,30 @@ describe("attachmentsRouter.upload", () => {
 		}
 	});
 
-	it("rejects unrecognized media type", async () => {
+	it("falls back to application/octet-stream for unrecognized media type", async () => {
 		const caller = createCaller();
-		await expect(
-			caller.upload({
-				data: { kind: "base64", data: PNG_BASE64 },
-				mediaType: "application/x-totally-fake",
-			}),
-		).rejects.toThrow(/unrecognized media type/i);
+		const result = await caller.upload({
+			data: { kind: "base64", data: PNG_BASE64 },
+			mediaType: "application/x-totally-fake",
+			originalFilename: "racetrack.bmad",
+		});
+		expect(result.mediaType).toBe("application/octet-stream");
+		const filePath = getAttachmentFilePath(
+			result.attachmentId,
+			"application/octet-stream",
+		);
+		expect(filePath).toMatch(/\.bin$/);
+		expect(existsSync(filePath)).toBe(true);
+	});
+
+	it("falls back to application/octet-stream for empty media type", async () => {
+		const caller = createCaller();
+		const result = await caller.upload({
+			data: { kind: "base64", data: PNG_BASE64 },
+			mediaType: "",
+			originalFilename: "racetrack.bmad",
+		});
+		expect(result.mediaType).toBe("application/octet-stream");
 	});
 
 	it("accepts a single decoded byte", async () => {
diff --git a/packages/host-service/src/trpc/router/attachments/attachments.ts b/packages/host-service/src/trpc/router/attachments/attachments.ts
index 59414349050..2b01619aed1 100644
--- a/packages/host-service/src/trpc/router/attachments/attachments.ts
+++ b/packages/host-service/src/trpc/router/attachments/attachments.ts
@@ -15,10 +15,12 @@ const uploadInputSchema = z.object({
 		kind: z.literal("base64"),
 		data: z.string().min(1),
 	}),
-	mediaType: z.string().min(1),
+	mediaType: z.string(),
 	originalFilename: z.string().optional(),
 });
 
+const FALLBACK_MEDIA_TYPE = "application/octet-stream";
+
 /**
  * Cheap size estimate from a base64 string without allocating the
  * decoded buffer. Used to reject oversized uploads before Buffer.from
@@ -36,12 +38,9 @@ export const attachmentsRouter = router({
 	 * renderer never sees the on-disk path.
 	 */
 	upload: protectedProcedure.input(uploadInputSchema).mutation(({ input }) => {
-		if (!mimeTypes.extension(input.mediaType)) {
-			throw new TRPCError({
-				code: "BAD_REQUEST",
-				message: `Unrecognized media type: ${input.mediaType}`,
-			});
-		}
+		const mediaType = mimeTypes.extension(input.mediaType)
+			? input.mediaType
+			: FALLBACK_MEDIA_TYPE;
 
 		// Reject before allocating the decoded buffer so a 1GB base64
 		// payload doesn't spike host memory only to be rejected at the end.
@@ -65,7 +64,7 @@ export const attachmentsRouter = router({
 
 		const metadata: AttachmentMetadata = {
 			attachmentId: randomUUID(),
-			mediaType: input.mediaType,
+			mediaType,
 			originalFilename: input.originalFilename,
 			sizeBytes: bytes.length,
 			createdAt: Date.now(),