Allow plugins to include Less files (#6051)

program/include/rcmail_output_html.php

@@ -1716,8 +1716,13 @@ EOF;
         ) {
             $css = '';
             foreach ($this->css_files as $file) {
-                $css .= html::tag('link', array('rel' => 'stylesheet',
-                    'type' => 'text/css', 'href' => $file, 'nl' => true));
+                $is_less = substr_compare($file, '.less', -5, 5, true) === 0;
+                $css    .= html::tag('link', array(
+                        'rel'  => $is_less ? 'stylesheet/less' : 'stylesheet',
+                        'type' => 'text/css',
+                        'href' => $file,
+                        'nl'   => true,
+                ));
             }
             $output = substr_replace($output, $css, $pos, 0);
         }

program/lib/Roundcube/rcube_plugin_api.php

@@ -611,8 +611,18 @@ class rcube_plugin_api
     {
         if (is_object($this->output) && $this->output->type == 'html') {
             if ($if_exists && $fn[0] != '/' && !preg_match('|^https?://|i', $fn)) {
-                $rcube = rcube::get_instance();
-                $path  = unslashify($rcube->config->get('assets_dir') ?: RCUBE_INSTALL_PATH);
+                $rcube      = rcube::get_instance();
+                $devel_mode = $rcube->config->get('devel_mode');
+                $assets_dir = $rcube->config->get('assets_dir');
+                $path       = unslashify($assets_dir ?: RCUBE_INSTALL_PATH);
+
+                // Prefer .less files in devel_mode (assume less.js is loaded)
+                if ($devel_mode) {
+                    $less = preg_replace('/\.css$/i', '.less', $fn);
+                    if ($less != $fn && is_file("$path/plugins/$less")) {
+                        $fn = $less;
+                    }
+                }
 
                 if (!is_file("$path/plugins/$fn")) {
                     return;