Merge branch 'attachment-handling' of https://github.com/thororm/PrivateBin into thororm-attachment-handling

apart from resolving conflicts:
- added missing docs
- inlined functions that were used in only one location
- updated unit test to support all previews
- fixed a regression that displayed the preview even when there was no preview and too early

lib/Configuration.php

@@ -54,7 +54,7 @@ class Configuration
             'urlshortener'             => '',
             'qrcode'                   => true,
             'icon'                     => 'identicon',
-            'cspheader'                => 'default-src \'none\'; manifest-src \'self\'; connect-src *; form-action \'none\'; script-src \'self\'; style-src \'self\'; font-src \'self\'; img-src \'self\' data:; referrer no-referrer; sandbox allow-same-origin allow-scripts allow-forms allow-popups',
+            'cspheader'                => 'default-src \'none\'; manifest-src \'self\'; connect-src *; script-src \'self\'; style-src \'self\'; font-src \'self\'; img-src \'self\' data:; media-src data:; object-src data:; Referrer-Policy: \'no-referrer\'; sandbox allow-same-origin allow-scripts allow-forms allow-popups',
             'zerobincompatibility'     => false,
         ),
         'expire' => array(