code: func (api UsersAPI) UpdateAvatarFile(w http.ResponseWriter, r *http.Request) {
  	oldLabel := mux.Vars(r)["label"]
  	newLabel := mux.Vars(r)["newlabel"]
  	username := mux.Vars(r)["username"]
  
  	userMgr := user.NewManager(r)
  	oldAvatar, errorOccurred := validateAvatarUpdateLabels(oldLabel, newLabel, username, userMgr, w)
  	if errorOccurred {
  		return
  	}
  
  	link, errorOccurred := saveMultiPartAvatarFile(w, r, userMgr)
  	if errorOccurred {
  		return
  	}
  
  	newAvatar := &user.Avatar{
  		Label:  newLabel,
  		Source: link,
  	}
  
  	replaceAvatar(w, r, oldAvatar, newAvatar, username, userMgr)
  }
docs: // UpdateAvatarFile is the handler for PUT /users/{username}/avatar/{label}/to/{newlabel}
  // Update the avatar and possibly the avatar file stored on itsyou.online