File formats, methods, and computer program products are provided for representing a workbook in a modular content framework. The modular content framework may include a file format container associated with modular parts. A file format includes logically separate modular parts that are associated with each other by one or more relationships where each modular part is associated with a relationship type. The modular parts include a workbook part operative as a guide for properties of the workbook and a worksheet part associated with the workbook part and operative to specify a definition of cells within a worksheet associated with the worksheet part. The modular parts may also include a document properties part containing built-in properties associated with the file format and a thumbnail part containing associated thumbnails. Each modular part is capable of being interrogated separately, extracted from the workbook, and/or reused in a different workbook.