There is only one file type that uses the .XLSMHTML extension:
Microsoft Excel MIME HTML Spreadsheet
An XLSMHTML file is an archived MIME HTML spreadsheet saved by Microsoft Excel (2007 and earlier), a common spreadsheet application. It contains HTML code that represents the spreadsheet in a format similar to the original .XLS file. XLSMHTML files are used for viewing published spreadsheets in a web browser.
Microsoft Excel no longer supports the XLSMHTML format. Instead of saving spreadsheets as XLSMHTML files, you can save them as .MHTML and .MHT files, which are more widely supported.
NOTE: To create a webpage version of your document in Excel 365, select, then choose the "Single File Web Page (*.mht, *.mhtml)" option from the dropdown menu.
Unfortunately, there are no known applications that fully support the XLSMHTML format since Microsoft Excel dropped support for it.
Updated: March 18, 2021