Split-out bambu-specific elements to their own 3MF 'variant'.

part of CURA-12099

plugins/3MFWriter/DefaultVariant.py

@@ -0,0 +1,33 @@
+#  Copyright (c) 2025 UltiMaker
+#  Cura is released under the terms of the LGPLv3 or higher.
+
+import xml.etree.ElementTree as ET
+import zipfile
+
+from PyQt6.QtCore import QBuffer
+from PyQt6.QtGui import QImage
+
+from .ThreeMFVariant import ThreeMFVariant
+
+# Standard 3MF paths
+METADATA_PATH = "Metadata"
+THUMBNAIL_PATH = f"{METADATA_PATH}/thumbnail.png"
+
+class DefaultVariant(ThreeMFVariant):
+    """Default implementation of the 3MF format."""
+    
+    @property
+    def mime_type(self) -> str:
+        return "application/vnd.ms-package.3dmanufacturing-3dmodel+xml"
+    
+    def process_thumbnail(self, snapshot: QImage, thumbnail_buffer: QBuffer, 
+                          archive: zipfile.ZipFile, relations_element: ET.Element) -> None:
+        """Process the thumbnail for default 3MF variant."""
+        thumbnail_file = zipfile.ZipInfo(THUMBNAIL_PATH)
+        # Don't try to compress snapshot file, because the PNG is pretty much as compact as it will get
+        archive.writestr(thumbnail_file, thumbnail_buffer.data())
+
+        # Add thumbnail relation to _rels/.rels file
+        ET.SubElement(relations_element, "Relationship",
+                      Target="/" + THUMBNAIL_PATH, Id="rel1",
+                      Type="http://schemas.openxmlformats.org/package/2006/relationships/metadata/thumbnail")