MSG_File_support.php

/*
* The following functions will add additional file types option to your file upload element
* In this case, You can enable MSG file format
* Just add this snippet to your theme's functions.php file or relevant place.
*/

add_filter('fluentform_file_type_options', function ($types) {
    $types[] = [
        'label' => __('MSG file - MSG', 'fluentform'),
        'value' => 'msg',
    ];
    return $types;
});

add_filter('upload_mimes', function ($mime_types) {
    $mime_types ['msg'] = 'application/vnd.ms-outlook';

    return $mime_types;
}, 1, 1);
add_action('fluentform_starting_file_upload', function () {
    add_filter('fluentform_uploader_args', function ($args) {
        $args['test_type'] = false;
        return $args;
    });
});

//http req
add_filter('ff_default_file_extensions', function ($mime_types) {
    $newTypes = [
        'application/vnd.ms-outlook' => 'msg',
    ];
    $mime_types = array_merge($mime_types, $newTypes);
    return $mime_types;
}, 1, 1);