One of our test Excel versions is:

The installation of the addin fails with the following error:
The file format and extension of the file don't match.
The file could be corrupted or unsafe.
Unless you trust its source, don't open it.
Do you want to open it anyway?
Looking at the old VBScript installer, it seems that the 64 bit version is being incorrectly chosen. The logic is this:
if Mid(excel.ProductCode, 21, 1) = "0" then
addInFile = addInFile & "\OpenFin.ExcelApi-AddIn.xll"
else
addInFile = addInFile & "\OpenFin.ExcelApi-AddIn64.xll"
end if
I assume it's similar in the EXE installer. However, our excel.ProductCode is {9AC08E99-230B-47e8-9721-4577B7F124EA}, so 64 bit is chosen, even though the Excel is 32 bit.
One of our test Excel versions is:

The installation of the addin fails with the following error:
Looking at the old VBScript installer, it seems that the 64 bit version is being incorrectly chosen. The logic is this:
I assume it's similar in the EXE installer. However, our
excel.ProductCodeis{9AC08E99-230B-47e8-9721-4577B7F124EA}, so 64 bit is chosen, even though the Excel is 32 bit.