When a document (PPTX, DOCX, XLSX, ODP, ODT, ODS) is attached,
extracted slide images were left orphaned after removing the
parent text entry. This adds a sourceDocumentId field to
AttachedFile that links all entries from the same document
extraction, and cascades removal of all entries in the group
when any one is removed.
Fixes#2426
Co-authored-by: chiamsun <chiamsun@users.noreply.github.com>
