Replaced direct file links with a JavaScript-based viewer
(`openAttachmentViewer`) and added a DevExtreme Popup
(`attachment-viewer-popup`) for displaying attachments in a
modal dialog. The viewer supports PDFs (via PDF.js), XML
(with CodeMirror syntax highlighting), plain text, images,
and provides a fallback for unsupported file types.
Dynamically load CodeMirror for XML files and handle errors
gracefully when loading file content. Added `onAttachmentPopupHiding`
to clear popup content on close.
Updated `ViewAttachmentModel` to return text/XML content
directly for AJAX requests, improving frontend performance
and enabling dynamic content loading.
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