[Media] Support any kind of thumbnails in the Core

Sanitize Attachments instead of Validate (part 1)
Ensure the intended filetypes and mimetypes during Vips conversions (part 1)
Various bug fixes

components/Posting/Posting.php

@@ -114,7 +114,7 @@ END;
         ]);
 
         $processed_attachments = [];
-        foreach ($attachments as $f) {
+        foreach ($attachments as $f) { // where $f is a Symfony\Component\HttpFoundation\File\UploadedFile
             $processed_attachments[] = GSFile::validateAndStoreFileAsAttachment(
                 $f, Common::config('attachments', 'dir'),
                 Security::sanitize($f->getClientOriginalName()),