v0.6.0

Added

• Excel Spreadsheet Interactive Preview Engine:
  • ExcelParserService: Memory-safe .xlsx OpenXML parser extracting sheet names (xl/workbook.xml), shared string lookup tables (xl/sharedStrings.xml), and row/column data matrices capped at 100 rows x 50 columns.
  • ExcelPreviewHandler: Handler supporting .xlsx and .xls attachments, returning multi-sheet workbook structure and metadata.
  • Template/file/excel_preview.php: Multi-sheet tabbed spreadsheet modal view with worksheet navigation tabs (Sheet1, Sheet2), A/B/C column headers, row index gutter, cell HTML entity escaping, and legacy format / truncation banners.
  • Added 'xlsx' and 'xls' extensions to attachment dropdown whitelist (Template/file/dropdown.php) with 5 MB file size caps in FileValidationService.
• Expanded Test Suite:
  • 39 new unit & integration tests covering OpenXML spreadsheet parsing, multi-sheet tab rendering, legacy .xls notification, XSS cell escaping, and 5 MB size cap enforcement (total 279 tests, 925 assertions).

Fixed

• Registered ExcelPreviewHandler in FilePreviewController handler registry (7-handler registry order).

Changed

• Updated Plugin.php version to 0.6.0.