@alivedise I was planning to let the devs, if who want to use their own mime-to-extension/extension-to-mime maps instead of the default maps when they use MimeMapper, as parameters like MimeMapper(mymap1, mymap2) to replace the default one, but I think now we should use the predefined maps to control the supported formats and types in all apps in gaia, so yes a singleton might be a suitable choice to achieve this.