MDL-80150 resource: Reintroduce file icon

The new activity card design, planned for Moodle 4.3, reintroduces
the resource MIME icon that was removed in Moodle 4.0.
This is a backport of MDL-78285.

mod/resource/lib.php

@@ -227,6 +227,7 @@ function resource_get_coursemodule_info($coursemodule) {
     if (count($files) >= 1) {
         $mainfile = reset($files);
         $resource->mainfile = $mainfile->get_filename();
+        $info->icon = file_file_icon($mainfile, 24);
     }
 
     $display = resource_get_final_display_type($resource);

mod/resource/tests/lib_test.php

@@ -144,11 +144,13 @@ class lib_test extends \advanced_testcase {
         $info = resource_get_coursemodule_info(
                 $DB->get_record('course_modules', array('id' => $resource2->cmid)));
         $this->assertEquals('R2', $info->name);
+        $this->assertEquals('f/text-24', $info->icon);
 
         // For third one, it should use the highest sortorder icon.
         $info = resource_get_coursemodule_info(
                 $DB->get_record('course_modules', array('id' => $resource3->cmid)));
         $this->assertEquals('R3', $info->name);
+        $this->assertEquals('f/document-24', $info->icon);
     }
 
     public function test_resource_core_calendar_provide_event_action() {