Configuration / upload.extensions
This option should contain a comma separated list of valid extensions that can be uploaded. Also this will further restrict the filtering of the filesystem.extensions config option. So you can for example let the user list more file types than the user can upload. This is especially useful when you want to restict image uploads only to a specific directory.
Remember that in addition to this setting, all file extensions must also be allowed in the filesystem.extensions setting.
$moxieManagerConfig['upload.extensions'] = 'gif,jpg,png';
<add key="upload.extensions" value="gif,jpg,png" />