All reports that have more than 50,000 records are downloaded as TXT and not as XSLX.
This record limitation is related to the lost of performance related to the old file format (xls).
Is possible to add a list of output format?
The available formats are in the Parameters table in Console --> Configuration --> Parameters --> Transform --> Report Settings. Report Max Rows Default is 50k that can be changed... and file format has a drop down.