Add support for showing AVIF images

This won't work in 100% of desktops, as old Electron doesn't support it,
but it will work in 99+%, and it's not a big deal for the few desktops
where it doesn't (it's an equally bad experience to today, where the
data still isn't usefully visible).

Author: pimterry

src/model/events/content-types.ts

@@ -75,6 +75,7 @@ const mimeTypeToContentTypeMap: { [mimeType: string]: ViewableContentType } = {
     'image/svg': 'image',
     'image/tiff': 'image',
     'image/webp': 'image',
+    'image/avif': 'image',
     'image/x-icon': 'image',
     'image/vnd.microsoft.icon': 'image',