Saving Excel Report returned from API call

[cbyrne]

I'm trying to save an excel report returned after calling /rest/latest/excel-reports/download/{XID}

My first attempt doubles the file size so it looks like I'm saving bytes as chars.

        saveExcelFile(data, report_filename) {
            let link = document.createElement("a");
            let blob = new Blob([data], {type: 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet'})
            link.href = URL.createObjectURL(blob);
            link.download = report_filename;
            link.click();
            URL.revokeObjectURL(link.href)
            link.remove();
        }

The response payload looks to be base64 encoded but calling atob fails.

I've also played around with a string to array buffer function to convert the data which gives me the correct filesize but still not a completely correct conversion.
https://stackoverflow.com/questions/34993292/how-to-save-xlsx-data-to-file-as-a-blob

A comparison of the hexdumps: (working file on left, file from original code snippet above on right)

From the hexdump I can see that the first ~49 bytes are are correct but then the encoding changes