Fetch all msg headers also for images to always get Content-Location

Previously all headers were only fetched for message/rfc822, or
if the Content-Type's "name" parameter was set, or if a Content-ID was
set.
The RFC doesn't require neither the "name" parameter nor a Content-ID
for using Content-Location, though, so we shouldn't depend on those.

Instead now all headers are also fetched if the main part of the
Content-Type is "image", to catch more cases.

program/lib/Roundcube/rcube_imap.php

@@ -2230,9 +2230,9 @@ protected function structure_part($part, $count = 0, $parent = '', $mime_headers
             }
         }
 
-        // fetch message headers if message/rfc822 or named part (could contain Content-Location header)
+        // fetch message headers if message/rfc822 or image or named part (could contain Content-Location header)
         if (
-            $struct->ctype_primary == 'message'
+            $struct->ctype_primary == 'message' || $struct->ctype_primary == 'image'
             || (!empty($struct->ctype_parameters['name']) && !empty($struct->content_id))
         ) {
             if (empty($mime_headers)) {