diff --git a/src/features/workspaces/files/workspace-file-processor.test.ts b/src/features/workspaces/files/workspace-file-processor.test.ts
new file mode 100644
index 000000000..b064e7737
--- /dev/null
+++ b/src/features/workspaces/files/workspace-file-processor.test.ts
@@ -0,0 +1,66 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+import { requestWorkspaceFileProcessor } from "#/features/workspaces/files/workspace-file-processor";
+
+const getRandom = vi.hoisted(() => vi.fn());
+
+vi.mock("@cloudflare/containers", () => ({
+	Container: class {},
+	getRandom,
+}));
+
+describe("requestWorkspaceFileProcessor", () => {
+	beforeEach(() => {
+		getRandom.mockReset();
+	});
+
+	it("declares a known content length so Workers can stream the R2 body", async () => {
+		const fetch = vi.fn().mockResolvedValue(new Response(null, { status: 204 }));
+		getRandom.mockResolvedValue({
+			fetch,
+			startAndWaitForPorts: vi.fn().mockResolvedValue(undefined),
+		});
+
+		await requestWorkspaceFileProcessor({} as Cloudflare.Env, {
+			body: streamOf(new Uint8Array([1, 2, 3])),
+			contentType: "application/pdf",
+			path: "/preview/pdf",
+			sizeBytes: 3,
+		});
+
+		expect(fetch).toHaveBeenCalledOnce();
+		const request = fetch.mock.calls[0]![0] as Request;
+		expect(request.headers.get("content-length")).toBe("3");
+		expect(request.headers.get("x-file-size")).toBe("3");
+		expect(request.headers.get("content-type")).toBe("application/pdf");
+	});
+
+	it("encodes the optional file name header", async () => {
+		const fetch = vi.fn().mockResolvedValue(new Response(null, { status: 204 }));
+		getRandom.mockResolvedValue({
+			fetch,
+			startAndWaitForPorts: vi.fn().mockResolvedValue(undefined),
+		});
+
+		await requestWorkspaceFileProcessor({} as Cloudflare.Env, {
+			body: streamOf(new Uint8Array([1])),
+			contentType: "image/png",
+			fileName: "my file.png",
+			path: "/preview/image",
+			sizeBytes: 1,
+		});
+
+		const request = fetch.mock.calls[0]![0] as Request;
+		expect(request.headers.get("x-file-name")).toBe("my%20file.png");
+	});
+});
+
+function streamOf(bytes: Uint8Array) {
+	const body = new Response(bytes.slice().buffer).body;
+
+	if (!body) {
+		throw new Error("Test stream was not created.");
+	}
+
+	return body;
+}
diff --git a/src/features/workspaces/files/workspace-file-processor.ts b/src/features/workspaces/files/workspace-file-processor.ts
index f738a1835..f900fca9b 100644
--- a/src/features/workspaces/files/workspace-file-processor.ts
+++ b/src/features/workspaces/files/workspace-file-processor.ts
@@ -32,6 +32,9 @@ export async function requestWorkspaceFileProcessor(
 	});
 
 	const headers = new Headers({
+		// Cloudflare Workers refuses to send a streamed request body without a known
+		// length, so declare it up front from the source object's verified size.
+		"content-length": String(input.sizeBytes),
 		"content-type": input.contentType,
 		"x-file-size": String(input.sizeBytes),
 	});