MM-10188: expect io.Reader in FileBackend.WriteFile (#8765)

This is a reworked set of changes originally from @josephGuo to begin
reducing the duplicated memory required when uploading files.

api/emoji_test.go

@@ -272,7 +272,7 @@ func TestDeleteEmoji(t *testing.T) {
 func createTestEmoji(t *testing.T, a *app.App, emoji *model.Emoji, imageData []byte) *model.Emoji {
 	emoji = store.Must(a.Srv.Store.Emoji().Save(emoji)).(*model.Emoji)
 
-	if err := a.WriteFile(imageData, "emoji/"+emoji.Id+"/image"); err != nil {
+	if _, err := a.WriteFile(bytes.NewReader(imageData), "emoji/"+emoji.Id+"/image"); err != nil {
 		store.Must(a.Srv.Store.Emoji().Delete(emoji.Id, time.Now().Unix()))
 		t.Fatalf("failed to write image: %v", err.Error())
 	}