The ornamental design for a windowframe having a mosquito net, as shown and described.
1. Window frame having a mosquito net
1.1 Perspective view showing the window frame having a mosquito net in a closed position.
1.2 Perspective view showing the window frame having a mosquito net in an open position.
The mosquito net and net frame illustrated in orange/red form part of the claimed design. The window frame, box and glass illustrated in white, grey and light blue form no part of the claimed design. No particular color shown forms part of the claimed design.