I finally got a chance to post my spreadsheet, and I see that the limits for .xls, .xlsx are very small (200 KB) while other files are up to 7 MB. For spreadsheets, this is too small and will limit this to all but the simplest spreadsheets. Mine, for example (with the equations and support data, and minimal sample data) comes in at almost 700 KB.
May I suggest increasing the limits to be closer to that of the various image formats?