diff --git a/CHANGELOG.md b/CHANGELOG.md
index 56a297a8..a971597c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,8 @@
 * *(pushrules)* Added support for `sender_notification_permission` condition
   kind (used for `@room` mentions).
 * *(crypto)* Added support for `json.RawMessage` in `EncryptMegolmEvent`.
+* *(mediaproxy)* Added `GetMediaResponseCallback` to write proxied response
+  directly instead of having to use an `io.Reader`.
 
 [MSC2781]: https://github.com/matrix-org/matrix-spec-proposals/pull/2781
 
diff --git a/mediaproxy/mediaproxy.go b/mediaproxy/mediaproxy.go
index f2591428..d1ab0815 100644
--- a/mediaproxy/mediaproxy.go
+++ b/mediaproxy/mediaproxy.go
@@ -32,20 +32,54 @@ type GetMediaResponse interface {
 	isGetMediaResponse()
 }
 
-func (*GetMediaResponseURL) isGetMediaResponse()  {}
-func (*GetMediaResponseData) isGetMediaResponse() {}
+func (*GetMediaResponseURL) isGetMediaResponse()      {}
+func (*GetMediaResponseData) isGetMediaResponse()     {}
+func (*GetMediaResponseCallback) isGetMediaResponse() {}
 
 type GetMediaResponseURL struct {
 	URL       string
 	ExpiresAt time.Time
 }
 
+type GetMediaResponseWriter interface {
+	GetMediaResponse
+	io.WriterTo
+	GetContentType() string
+	GetContentLength() int64
+}
+
 type GetMediaResponseData struct {
 	Reader        io.ReadCloser
 	ContentType   string
 	ContentLength int64
 }
 
+func (d *GetMediaResponseData) WriteTo(w io.Writer) (int64, error) {
+	return io.Copy(w, d.Reader)
+}
+
+func (d *GetMediaResponseData) GetContentType() string {
+	return d.ContentType
+}
+
+func (d *GetMediaResponseData) GetContentLength() int64 {
+	return d.ContentLength
+}
+
+type GetMediaResponseCallback struct {
+	Callback      func(w io.Writer) (int64, error)
+	ContentType   string
+	ContentLength int64
+}
+
+func (d *GetMediaResponseCallback) WriteTo(w io.Writer) (int64, error) {
+	return d.Callback(w)
+}
+
+func (d *GetMediaResponseCallback) GetContentLength() int64 {
+	return d.ContentLength
+}
+
 type GetMediaFunc = func(ctx context.Context, mediaID string) (response GetMediaResponse, err error)
 
 type MediaProxy struct {
@@ -316,21 +350,21 @@ func (mp *MediaProxy) DownloadMediaFederation(w http.ResponseWriter, r *http.Req
 			log.Err(err).Msg("Failed to create multipart redirect field")
 			return
 		}
-	} else if dataResp, ok := resp.(*GetMediaResponseData); ok {
+	} else if dataResp, ok := resp.(GetMediaResponseWriter); ok {
 		dataPart, err := mpw.CreatePart(textproto.MIMEHeader{
-			"Content-Type": {dataResp.ContentType},
+			"Content-Type": {dataResp.GetContentType()},
 		})
 		if err != nil {
 			log.Err(err).Msg("Failed to create multipart data field")
 			return
 		}
-		_, err = io.Copy(dataPart, dataResp.Reader)
+		_, err = dataResp.WriteTo(dataPart)
 		if err != nil {
 			log.Err(err).Msg("Failed to write multipart data field")
 			return
 		}
 	} else {
-		panic("unknown GetMediaResponse type")
+		panic(fmt.Errorf("unknown GetMediaResponse type %T", resp))
 	}
 	err = mpw.Close()
 	if err != nil {
@@ -374,18 +408,18 @@ func (mp *MediaProxy) DownloadMedia(w http.ResponseWriter, r *http.Request) {
 			w.Header().Set("Cache-Control", "no-store")
 		}
 		w.WriteHeader(http.StatusTemporaryRedirect)
-	} else if dataResp, ok := resp.(*GetMediaResponseData); ok {
-		w.Header().Set("Content-Type", dataResp.ContentType)
-		if dataResp.ContentLength != 0 {
-			w.Header().Set("Content-Length", strconv.FormatInt(dataResp.ContentLength, 10))
+	} else if dataResp, ok := resp.(GetMediaResponseWriter); ok {
+		w.Header().Set("Content-Type", dataResp.GetContentType())
+		if dataResp.GetContentLength() != 0 {
+			w.Header().Set("Content-Length", strconv.FormatInt(dataResp.GetContentLength(), 10))
 		}
 		w.WriteHeader(http.StatusOK)
-		_, err := io.Copy(w, dataResp.Reader)
+		_, err := dataResp.WriteTo(w)
 		if err != nil {
 			log.Err(err).Msg("Failed to write media data")
 		}
 	} else {
-		panic("unknown GetMediaResponse type")
+		panic(fmt.Errorf("unknown GetMediaResponse type %T", resp))
 	}
 }