diff --git a/src/matrix/SessionContainer.js b/src/matrix/SessionContainer.js
index 89325d49..7b0dfa1e 100644
--- a/src/matrix/SessionContainer.js
+++ b/src/matrix/SessionContainer.js
@@ -22,7 +22,7 @@ import {ObservableValue} from "../observable/ObservableValue";
 import {HomeServerApi} from "./net/HomeServerApi.js";
 import {Reconnector, ConnectionStatus} from "./net/Reconnector.js";
 import {ExponentialRetryDelay} from "./net/ExponentialRetryDelay";
-import {MediaRepository} from "./net/MediaRepository.js";
+import {MediaRepository} from "./net/MediaRepository";
 import {RequestScheduler} from "./net/RequestScheduler.js";
 import {Sync, SyncStatus} from "./Sync.js";
 import {Session} from "./Session.js";
diff --git a/src/matrix/net/MediaRepository.js b/src/matrix/net/MediaRepository.ts
similarity index 71%
rename from src/matrix/net/MediaRepository.js
rename to src/matrix/net/MediaRepository.ts
index a7875d99..0bf70149 100644
--- a/src/matrix/net/MediaRepository.js
+++ b/src/matrix/net/MediaRepository.ts
@@ -16,14 +16,20 @@ limitations under the License.
 
 import {encodeQueryParams} from "./common";
 import {decryptAttachment} from "../e2ee/attachment.js";
+import {Platform} from "../../platform/web/Platform.js";
+import {BlobHandle} from "../../platform/web/dom/BlobHandle.js";
+import type {IAttachment, IEncryptedFile} from "./types/response";
 
 export class MediaRepository {
-    constructor({homeserver, platform}) {
+    private readonly _homeserver: string;
+    private readonly _platform: Platform;
+
+    constructor({homeserver, platform}: {homeserver:string, platform: Platform}) {
         this._homeserver = homeserver;
         this._platform = platform;
     }
 
-    mxcUrlThumbnail(url, width, height, method) {
+    mxcUrlThumbnail(url: string, width: number, height: number, method: "crop" | "scale"): string | null {
         const parts = this._parseMxcUrl(url);
         if (parts) {
             const [serverName, mediaId] = parts;
@@ -33,7 +39,7 @@ export class MediaRepository {
         return null;
     }
 
-    mxcUrl(url) {
+    mxcUrl(url: string): string | null {
         const parts = this._parseMxcUrl(url);
         if (parts) {
             const [serverName, mediaId] = parts;
@@ -43,7 +49,7 @@ export class MediaRepository {
         }
     }
 
-    _parseMxcUrl(url) {
+    _parseMxcUrl(url: string): string[] | null {
         const prefix = "mxc://";
         if (url.startsWith(prefix)) {
             return url.substr(prefix.length).split("/", 2);
@@ -52,24 +58,24 @@ export class MediaRepository {
         }
     }
 
-    async downloadEncryptedFile(fileEntry, cache = false) {
+    async downloadEncryptedFile(fileEntry: IEncryptedFile, cache: boolean = false): Promise<BlobHandle> {
         const url = this.mxcUrl(fileEntry.url);
         const {body: encryptedBuffer} = await this._platform.request(url, {method: "GET", format: "buffer", cache}).response();
         const decryptedBuffer = await decryptAttachment(this._platform, encryptedBuffer, fileEntry);
         return this._platform.createBlob(decryptedBuffer, fileEntry.mimetype);
     }
 
-    async downloadPlaintextFile(mxcUrl, mimetype, cache = false) {
+    async downloadPlaintextFile(mxcUrl: string, mimetype: string, cache: boolean = false): Promise<BlobHandle> {
         const url = this.mxcUrl(mxcUrl);
         const {body: buffer} = await this._platform.request(url, {method: "GET", format: "buffer", cache}).response();
         return this._platform.createBlob(buffer, mimetype);
     }
 
-    async downloadAttachment(content, cache = false) {
+    async downloadAttachment(content: IAttachment, cache: boolean = false): Promise<BlobHandle> {
         if (content.file) {
             return this.downloadEncryptedFile(content.file, cache);
         } else {
-            return this.downloadPlaintextFile(content.url, content.info?.mimetype, cache);
+            return this.downloadPlaintextFile(content.url!, content.info?.mimetype, cache);
         }
     }
 }
diff --git a/src/matrix/net/types/response.ts b/src/matrix/net/types/response.ts
new file mode 100644
index 00000000..762bd683
--- /dev/null
+++ b/src/matrix/net/types/response.ts
@@ -0,0 +1,54 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+export interface IAttachment {
+    body: string;
+    info: IAttachmentInfo;
+    // todo: what about m.audio?
+    msgtype: "m.image" | "m.file" | "m.video";
+    url?: string;
+    file?: IEncryptedFile;
+    filename?: string;
+}
+
+export interface IEncryptedFile {
+    key: JsonWebKey;
+    iv: string;
+    hashes: {
+        sha256: string;
+    };
+    url: string;
+    v: string;
+    mimetype?: string;
+}
+
+interface IAttachmentInfo {
+    h?: number;
+    w?: number;
+    mimetype: string;
+    size: number;
+    duration?: number;
+    thumbnail_url?: string;
+    thumbnail_file?: IEncryptedFile;
+    thumbnail_info?: IThumbnailInfo;
+}
+
+interface IThumbnailInfo {
+    h: number;
+    w: number;
+    mimetype: string;
+    size: number;
+}