diff --git a/program/actions/mail/show.php b/program/actions/mail/show.php
index efe6a481aa0..b5e1de64afc 100644
--- a/program/actions/mail/show.php
+++ b/program/actions/mail/show.php
@@ -204,7 +204,7 @@ public static function message_attachments($attrib)
             }
 
             // Skip inline images
-            if (strpos($mimetype, 'image/') === 0 && !self::is_attachment(self::$MESSAGE, $attach_prop)) {
+            if (strpos($mimetype, 'image/') === 0 && self::$MESSAGE->is_referred_attachment($attach_prop)) {
                 continue;
             }
 
@@ -621,7 +621,10 @@ public static function message_full_headers($attrib)
      */
     public static function message_body($attrib)
     {
-        if (empty(self::$MESSAGE) || (empty(self::$MESSAGE->parts) && empty(self::$MESSAGE->body))) {
+        // Exit early if there's no content to be shown anyway.
+        // `mime_parts` also includes a message's body, even if it originally
+        // was the only part of the message.
+        if (empty(self::$MESSAGE) || empty(self::$MESSAGE->mime_parts)) {
             return '';
         }
 
@@ -745,7 +748,7 @@ public static function message_body($attrib)
                 // Content-Type: image/*...
                 if ($mimetype = self::part_image_type($attach_prop)) {
                     // Skip inline images
-                    if (!self::is_attachment(self::$MESSAGE, $attach_prop)) {
+                    if (self::$MESSAGE->is_referred_attachment($attach_prop)) {
                         continue;
                     }
 
@@ -889,30 +892,4 @@ public static function mdn_request_handler($message)
             }
         }
     }
-
-    /**
-     * Check whether the message part is a normal attachment
-     *
-     * @param rcube_message      $message Message object
-     * @param rcube_message_part $part    Message part
-     *
-     * @return bool
-     */
-    protected static function is_attachment($message, $part)
-    {
-        // Inline attachment with Content-Id specified
-        if (!empty($part->content_id) && $part->disposition == 'inline') {
-            return false;
-        }
-
-        // Any image attached to multipart/related message (#7184)
-        $parent_id = preg_replace('/\.[0-9]+$/', '', $part->mime_id);
-        $parent = $message->mime_parts[$parent_id] ?? null;
-
-        if ($parent && $parent->mimetype == 'multipart/related') {
-            return false;
-        }
-
-        return true;
-    }
 }
diff --git a/program/lib/Roundcube/rcube_message.php b/program/lib/Roundcube/rcube_message.php
index eb6be0419c1..15a432f33f0 100644
--- a/program/lib/Roundcube/rcube_message.php
+++ b/program/lib/Roundcube/rcube_message.php
@@ -50,6 +50,14 @@ class rcube_message
     protected $got_html_part = false;
     protected $tnef_decode = false;
 
+    /**
+     * This holds a list of Content-IDs and Content-Locations by which parts of
+     * this message are referenced (e.g. in HTML parts).
+     *
+     * @var array
+     */
+    protected $replacement_references;
+
     public $uid;
     public $folder;
     public $headers;
@@ -569,6 +577,54 @@ public function is_attachment($part)
         return false;
     }
 
+    /**
+     * Get a cached list of replacement references, which are collected during
+     * parsing from Content-Id and Content-Location headers of mime-parts.
+     */
+    protected function get_replacement_references(): array
+    {
+        if ($this->replacement_references === null) {
+            $this->replacement_references = [];
+            foreach ($this->mime_parts as $mime_part) {
+                foreach ($mime_part->replaces as $key => $value) {
+                    $this->replacement_references[] = preg_replace('/^cid:/', '', $key);
+                }
+            }
+        }
+        return $this->replacement_references;
+    }
+
+    /**
+     * 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_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, $references)) {
+            return true;
+        }
+
+        // Filter out attachments that are reference by their
+        // Content-Location in another mime-part.
+        if (!empty($part->content_location) && in_array($part->content_location, $references)) {
+            return true;
+        }
+
+        return false;
+    }
+
     /**
      * In a multipart/encrypted encrypted message,
      * find the encrypted message payload part.