Rename method to more specific name

With this meaning we also get rid of the negation in the calling code.

program/actions/mail/show.php

@@ -204,7 +204,7 @@ public static function message_attachments($attrib)
             }
 
             // Skip inline images
-            if (strpos($mimetype, 'image/') === 0 && !self::$MESSAGE->is_standalone_attachment($attach_prop)) {
+            if (strpos($mimetype, 'image/') === 0 && self::$MESSAGE->is_referred_attachment($attach_prop)) {
                 continue;
             }
 
@@ -745,7 +745,7 @@ public static function message_body($attrib)
                 // Content-Type: image/*...
                 if ($mimetype = self::part_image_type($attach_prop)) {
                     // Skip inline images
-                    if (!self::$MESSAGE->is_standalone_attachment($attach_prop)) {
+                    if (self::$MESSAGE->is_referred_attachment($attach_prop)) {
                         continue;
                     }
 

program/lib/Roundcube/rcube_message.php

@@ -595,34 +595,34 @@ protected function get_replacement_references(): array
     }
 
     /**
-     * Checks if a given message part is a "standalone" attachment, which means
-     * that our code is responsible to show it (either in/after the message
-     * body or as downloadable file).
-     * Technically this is determined by checking if the part is referenced in
-     * any HTML-part of the message.
+     * Checks if a given message part is referred to from another message part.
+     * Usually this happens if an HTML-part includes images to show inline, but
+     * technically there can be other cases, too.
+     * In any case, an attachment that is *not* referred to, shall be shown to
+     * the users (either in/after the message body or as downloadable file).
      *
      * @param rcube_message_part $part Message part
      *
      * @return bool True if the part is an attachment part
      */
-    public function is_standalone_attachment(rcube_message_part $part): bool
+    public function is_referred_attachment(rcube_message_part $part): bool
     {
         $references = $this->get_replacement_references();
 
         // This code is intentionally verbose to keep it comprehensible.
         // Filter out attachments that are reference by their Content-ID in
         // another mime-part.
         if (!empty($part->content_id) && in_array($part->content_id, $references)) {
-            return false;
+            return true;
         }
 
         // Filter out attachments that are reference by their
         // Content-Location in another mime-part.
         if (!empty($part->content_location) && in_array($part->content_location, $references)) {
-            return false;
+            return true;
         }
 
-        return true;
+        return false;
     }
 
     /**