diff --git a/src/platform/web/dom/request/common.js b/src/platform/web/dom/request/common.js
index d97456aa..c073cf6b 100644
--- a/src/platform/web/dom/request/common.js
+++ b/src/platform/web/dom/request/common.js
@@ -27,6 +27,21 @@ export function addCacheBuster(urlStr, random = Math.random) {
     return urlStr + `_cacheBuster=${Math.ceil(random() * Number.MAX_SAFE_INTEGER)}`;
 }
 
+export function mapAsFormData(map) {
+    const formData = new FormData();
+    for (const [name, value] of map) {
+        let filename;
+        // Special case {name: string, blob: BlobHandle} to set a filename.
+        // This is the format returned by platform.openFile
+        if (value.blob?.nativeBlob && value.name) {
+            formData.set(name, value.blob.nativeBlob, value.name);
+        } else {
+            formData.set(name, value);
+        }
+    }
+    return formData;
+}
+
 export function tests() {
     return {
         "add cache buster": assert => {
diff --git a/src/platform/web/dom/request/fetch.js b/src/platform/web/dom/request/fetch.js
index 1d2ef3c0..d50f6194 100644
--- a/src/platform/web/dom/request/fetch.js
+++ b/src/platform/web/dom/request/fetch.js
@@ -20,7 +20,7 @@ import {
     ConnectionError
 } from "../../../../matrix/error.js";
 import {abortOnTimeout} from "../../../../utils/timeout";
-import {addCacheBuster} from "./common.js";
+import {addCacheBuster, mapAsFormData} from "./common.js";
 import {xhrRequest} from "./xhr.js";
 
 class RequestResult {
@@ -71,18 +71,7 @@ export function createFetchRequest(createTimeout, serviceWorkerHandler) {
             body = body.nativeBlob;
         }
         if (body instanceof Map) {
-            const formData = new FormData();
-            for (const [name, value] of body) {
-                let filename;
-                // Special case {name: string, blob: BlobHandle} to set a filename.
-                // This is the format returned by platform.openFile
-                if (value.blob?.nativeBlob && value.name) {
-                    formData.set(name, value.blob.nativeBlob, value.name);
-                } else {
-                    formData.set(name, value);
-                }
-            }
-            body = formData;
+            body = mapAsFormData(body);
         }
         let options = {method, body};
         if (controller) {
diff --git a/src/platform/web/dom/request/xhr.js b/src/platform/web/dom/request/xhr.js
index c3ad8ae8..fba7123e 100644
--- a/src/platform/web/dom/request/xhr.js
+++ b/src/platform/web/dom/request/xhr.js
@@ -18,7 +18,7 @@ import {
     AbortError,
     ConnectionError
 } from "../../../../matrix/error.js";
-import {addCacheBuster} from "./common.js";
+import {addCacheBuster, mapAsFormData} from "./common.js";
 
 class RequestResult {
     constructor(promise, xhr) {
@@ -94,6 +94,9 @@ export function xhrRequest(url, options) {
     if (body?.nativeBlob) {
         body = body.nativeBlob;
     }
+    if (body instanceof Map) {
+        body = mapAsFormData(body);
+    }
     xhr.send(body || null);
 
     return new RequestResult(promise, xhr);