Mime type handling overhaul

Changed mime type map to deal with the reality that certain file types have multiple extensions and/or multiple mime types, as well as constants supporting all of the data. Created new functions using the updated mime type map to resolve mime types and extensions. Updated various items around the project that determine mime/extension to take advantage of the new functions.

ext/random_image/main.php

@@ -35,7 +35,7 @@ class RandomImage extends Extension
                 send_event(new DisplayingImageEvent($image));
             } elseif ($action === "widget") {
                 $page->set_mode(PageMode::DATA);
-                $page->set_type("text/html");
+                $page->set_type(MIME_TYPE_HTML);
                 $page->set_data($this->theme->build_thumb_html($image));
             }
         }