Fixed drawing handling on HTML to Markdown conversion

app/Entities/Tools/Markdown/CustomImageConverter.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace BookStack\Entities\Tools\Markdown;
+
+use League\HTMLToMarkdown\Converter\ImageConverter;
+use League\HTMLToMarkdown\ElementInterface;
+
+class CustomImageConverter extends ImageConverter
+{
+    public function convert(ElementInterface $element): string
+    {
+        $parent = $element->getParent();
+
+        // Remain as HTML if within diagram block.
+        $withinDrawing = $parent && !empty($parent->getAttribute('drawio-diagram'));
+        if ($withinDrawing) {
+            $src = e($element->getAttribute('src'));
+            $alt = e($element->getAttribute('alt'));
+            return "<img src=\"{$src}\" alt=\"{$alt}\"/>";
+        }
+
+        return parent::convert($element);
+    }
+}