Implement MSC3916 (#3397)

Needs https://github.com/matrix-org/gomatrixserverlib/pull/437

mediaapi/routing/download_test.go

@@ -1,8 +1,13 @@
 package routing
 
 import (
+	"bytes"
+	"io"
+	"net/http"
+	"net/http/httptest"
 	"testing"
 
+	"github.com/matrix-org/dendrite/mediaapi/types"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -11,3 +16,28 @@ func Test_dispositionFor(t *testing.T) {
 	assert.Equal(t, "attachment", contentDispositionFor("image/svg"), "image/svg")
 	assert.Equal(t, "inline", contentDispositionFor("image/jpeg"), "image/jpg")
 }
+
+func Test_Multipart(t *testing.T) {
+	r := &downloadRequest{
+		MediaMetadata: &types.MediaMetadata{},
+	}
+	data := bytes.Buffer{}
+	responseBody := "This media is plain text. Maybe somebody used it as a paste bin."
+	data.WriteString(responseBody)
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, err := multipartResponse(w, r, "text/plain", &data)
+		assert.NoError(t, err)
+	}))
+	defer srv.Close()
+
+	resp, err := srv.Client().Get(srv.URL)
+	assert.NoError(t, err)
+	defer resp.Body.Close()
+	// contentLength is always 0, since there's no Content-Length header on the multipart part.
+	err, _, reader := parseMultipartResponse(r, resp, 1000)
+	assert.NoError(t, err)
+	gotResponse, err := io.ReadAll(reader)
+	assert.NoError(t, err)
+	assert.Equal(t, responseBody, string(gotResponse))
+}