One of the differences between the new OOXML file formats (e.g. .xlsx, .xlsm, .docx, .docm) that were released with Office 2007 and the older binary file formats (eg. .xls, .doc) is that each OOXML file extension has a unique Multipurpose Internet Mail Extension (MIME) type. This is a departure from the MIME types for the binary file formats, where the same MIME type could apply to several file extensions.

For example, the binary “application/vnd.ms-excel” MIME type applies to the .xls, .xlt, and .xla file extensions, but OOXML .xlsx and .xltx have separate MIME types: “application/vnd.openxmlformats-officedocument.spreadsheetml.sheet” and “application/vnd.openxmlformats-officedocument.spreadsheetml.template,” respectively.

An unexpected issue

The change in MIME types can cause unexpected issues to occur. I recently helped a customer who was experiencing strange behavior in his upload application. In his application, he was using FileUp to upload files to a SQL server database, in addition to some information about the files. When the customer tried to upload an XLS file, the upload completed successfully, but if he tried to upload an XLSX file, he started getting error messages. Continue reading Unexpected Issues with the New Office 2007 File Format MIME types →