+ 'thumbnail' => [
+ 'dir' => null, // falls back to File::path('thumb') (equivalent to ['attachments']['dir'] . '/thumb/')
+ 'path' => null, // falls back to generating a URL with File::url('thumb/$filename') (equivalent to ['attachments']['path'] . '/thumb/')
+ 'server' => null, // Only used if ['thumbnail']['path'] is NOT empty, and then it falls back to ['site']['server'], schema is decided from GNUsocial::useHTTPS()
+
+ 'crop' => false, // overridden to true if thumb height === null