Filepicker..

Ok, so here it is:

1. Create the file "js/tinymce.plugin.js" inside the FileRun installation folder. Paste the following inside:

tinymce.PluginManager.add("filerun", function(editor) {
    function filerun_onMessage(event) {
        if(editor.settings.external_plugins.filerun.toLowerCase().indexOf(event.origin.toLowerCase()) === 0){
            if(event.data.sender === 'FileRun'){
                tinymce.activeEditor.insertContent(event.data.html);
                tinymce.activeEditor.windowManager.close();
                window.removeEventListener('message', filerun_onMessage, false);
            }
        }
    }
    function openFileRun() {
        var width = window.innerWidth - 20;
        var height = window.innerHeight - 40;
        if (width > 1800) {width = 1800;}
        if (height > 1200) {height = 1200;}
        if (width > 600) {
            var width_reduce = (width - 20) % 138;
            width = width - width_reduce + 10;
        }
        window.addEventListener("message", filerun_onMessage, false);
        var baseURL = editor.settings.external_plugins.filerun.replace('js/tinymce.plugin.js', '');
        editor.windowManager.open({
            title: editor.settings.filerun.window_title || "FileRun",
            file: baseURL,
            width: width, height: height,
            inline: true, resizable: false, maximizable: false
        });
    }
    editor.addShortcut("Ctrl+E", "", openFileRun);
    editor.addMenuItem("filerun", {
        icon: "browse",
        text: editor.settings.filerun.button_title || 'Insert file',
        shortcut: "Ctrl+E",
        onclick: openFileRun,
        context: "insert"
    });
});

2. Initialize your TinyMCE editor like this:

tinymce.init({
    selector:'textarea',
    plugins: 'image link filerun',
    filerun: {
        window_title: 'Insert File Path',
        button_title: 'Insert some file'
    },
    external_plugins: {
        'filerun': 'http://FILERUN-URL/js/tinymce.plugin.js'
    }
});

Step-by-step:

a) Add the "external_plugins" part. Replace "http://FileRun-URL/" with your actual URL.

b) Add "filerun" to the list of plugins.

c) Optional: the "filerun" settings are not mandatory

3. Create the file "customizables/plugins/tinymce_filepicker/app.php" inside your FileRun installation folder (Note: requires creating the empty folder "tinymce_filepicker".) Paste the following inside:

<?php
class custom_tinymce_filepicker extends \FileRun\Files\Plugin {
    static $localeSection = 'Custom Actions';
    function init() {
        $this->JSconfig = [
            "title" => self::t('Insert to TinyMCE'),
            'iconCls' => 'fa fa-fw fa-reply-all',
            'popup' => true
        ];
    }
    function isDisabled() {
        return !\FileRun\Perms::isSuperUser();
    }
    function run() {
        $pathInfo = \FileRun\Files\Utils::getPathInfo($this->data['relativePath']);
        $fullPath = $pathInfo['fullPath'];
        ?>
        <script>
            if (!window.parent) {
                document.write('This plugin is designed to be used from withing a TinyMCE editor!');
            } else {
                with (window.parent) {
                    window.parent.postMessage({
                        sender: 'FileRun',
                        html: '<?php echo \S::safeJS(\S::forHTML($fullPath));?>'
                    }, '*');
                }
            }
        </script>
    <?php
    }
}

How to use:

1. Clear your browser's cache.

2. Open your TinyMCE editor.

3. Select "Insert" -> "Insert some file"

4. A popup will open with FileRun

5. Browse to your desired file, right-click it and select "Open with.." -> "Insert to TinyMCE"

The FileRun popup will close and the absolute filesystem path of the selected file will be inserted in the editor.

Note: The FileRun "Insert to TinyMCE" plugin will be available only to the superuser. If you wish to change that, remove the entire "isDisabled" function from the code.