diff --git a/src/matrix/e2ee/attachment.js b/src/matrix/e2ee/attachment.js
index cea91cfa..02a051ed 100644
--- a/src/matrix/e2ee/attachment.js
+++ b/src/matrix/e2ee/attachment.js
@@ -57,13 +57,15 @@ export async function decryptAttachment(crypto, ciphertextBuffer, info) {
     return decryptedBuffer;
 }
 
-export async function encryptAttachment(crypto, data) {
+export async function encryptAttachment(platform, blob) {
+    const {crypto} = platform;
     const iv = await crypto.aes.generateIV();
     const key = await crypto.aes.generateKey("jwk", 256);
-    const ciphertext = await crypto.aes.encryptCTR({key, iv, data});
+    const buffer = await blob.readAsBuffer();
+    const ciphertext = await crypto.aes.encryptCTR({key, iv, data: buffer});
     const digest = await crypto.digest("SHA-256", ciphertext);
     return {
-        data: ciphertext,
+        blob: platform.createBlob(ciphertext, blob.mimeType),
         info: {
             v: "v2",
             key,