File Extensions

CHM	A compiled HTML help file
CSV	CSV (Comma delimited)
DBF	Standard database file used by the dBASE database management system
DIF	Data Interchange Format
DLL	A dynamic link library file used to contain code
HTM	Web Page
ODS	OpenDocument Spreadsheet
OLB	An object type library
PDF
PRM	Formatted Text (Space delimited)
SLK	Symbolic Link (sylk)
TXT	Text (Tab delimited), Unicode
WJI-3
WK1-4
WQ1
XLA	Add-in (macro enabled). Excel 97-2003
XLAM	Add-in (macro enabled, previously xla)
XLB	An Excel toolbar configuration file
XLC	An Excel 4.0 file
XLL	Link library, Excel DLL
XLS	Excel 97-2003 Workbook
XLSB	Binary workbook (useful for very large or complex workbooks and the Personal.xlsb)
XLSM	Workbook (macro enabled, previously .xls)
XLSX	Workbook (macro free, previously .xls). Strict Open XML Spreadsheet
XLT	Template. Excel 97-2003
XLTM	Template (macro enabled, previously .xlt)
XLTX	Template (macro enabled, previously .xlt)
XLW	Workspace
XML	XML Spreadsheet 2003
XPS	XPS Document

Strict Open XML Spreadsheet - Lets you read and write ISO8601 dates to resolve a leap year issue for the year 1900

The 2003 xml formats couldn't support/store vba code, charts and other graphic objects, custom views, drawing objects, layers, outlines, scenarios, shared workbooks