diff --git a/api.planx.uk/modules/file/service/uploadFile.ts b/api.planx.uk/modules/file/service/uploadFile.ts
index a6ee74227b..d272018c70 100644
--- a/api.planx.uk/modules/file/service/uploadFile.ts
+++ b/api.planx.uk/modules/file/service/uploadFile.ts
@@ -7,6 +7,7 @@ import mime from "mime";
 import { customAlphabet } from "nanoid";
 import { isLiveEnv } from "../../../helpers.js";
 import { s3Factory } from "./utils.js";
+import { Readable } from "stream";
 const nanoid = customAlphabet("1234567890abcdefghijklmnopqrstuvwxyz", 8);
 
 export const uploadPublicFile = async (
@@ -28,10 +29,14 @@ export const uploadPublicFile = async (
 };
 
 export const uploadPrivateFile = async (
-  file: Express.Multer.File,
+  file: Express.Multer.File | Buffer,
   filename: string,
   filekey?: string,
 ) => {
+  if (file instanceof Buffer) {
+    file = convertToMulterFile(file);
+  }
+
   const s3 = s3Factory();
 
   const { params, key, fileType } = generateFileParams(file, filename, filekey);
@@ -63,6 +68,19 @@ const buildFileUrl = async (key: string, path: "public" | "private") => {
   return `${process.env.API_URL_EXT}/file/${path}${s3Pathname}`;
 };
 
+const convertToMulterFile = (buffer: Buffer): Express.Multer.File => ({
+  buffer: buffer,
+  originalname: "${data.id}.json",
+  mimetype: "application/json",
+  size: buffer.length,
+  fieldname: "file",
+  encoding: "7bit",
+  stream: Readable.from(buffer),
+  destination: "",
+  filename: "",
+  path: "",
+});
+
 export function generateFileParams(
   file: Express.Multer.File,
   filename: string,
@@ -79,9 +97,9 @@ export function generateFileParams(
     ACL: "public-read",
     Bucket: process.env.AWS_S3_BUCKET,
     Key: key,
-    Body: file.buffer || JSON.stringify(file),
+    Body: file.buffer,
     ContentDisposition: `inline;filename="${filename}"`,
-    ContentType: file.mimetype || "application/json",
+    ContentType: file.mimetype,
   };
 
   return {
diff --git a/api.planx.uk/modules/send/s3/index.ts b/api.planx.uk/modules/send/s3/index.ts
index 6f7186ae8b..8bf46acf5e 100644
--- a/api.planx.uk/modules/send/s3/index.ts
+++ b/api.planx.uk/modules/send/s3/index.ts
@@ -9,6 +9,10 @@ import { markSessionAsSubmitted } from "../../saveAndReturn/service/utils.js";
 import { isApplicationTypeSupported } from "../utils/helpers.js";
 import type { SendIntegrationController } from "../types.js";
 
+// TS fails to validate this complex type when awaited
+// Represent as a simple object to allow enough typechecking to kick in
+type DigitalPlanningPayload = Record<string, unknown>;
+
 interface CreateS3Application {
   insertS3Application: {
     id: string;
@@ -45,15 +49,13 @@ const sendToS3: SendIntegrationController = async (_req, res, next) => {
 
     // Generate the ODP Schema JSON, skipping validation if not a supported application type
     const doValidation = isApplicationTypeSupported(passport);
-    const exportData = doValidation
-      ? await $api.export.digitalPlanningDataPayload(sessionId)
-      : await $api.export.digitalPlanningDataPayload(sessionId, true);
+    const exportData: DigitalPlanningPayload =
+      await $api.export.digitalPlanningDataPayload(sessionId, doValidation);
+
+    const buffer = Buffer.from(JSON.stringify(exportData));
 
     // Create and upload the data as an S3 file
-    const { fileUrl } = await uploadPrivateFile(
-      exportData,
-      `${sessionId}.json`,
-    );
+    const { fileUrl } = await uploadPrivateFile(buffer, `${sessionId}.json`);
 
     // Send a notification with the file URL to the Power Automate webhook
     const webhookRequest: AxiosRequestConfig = {