TSFReportMailer auto send attachment format .xls instead of the default .pdf

Using a task we send email messages together with attachment report which is auto generated, using TSFReportMailer.

This works fine, our message can be created, along with subject and recipient(s), and even sends an attachment, however, so far only in .pdf-format.

According to the documentation, here https://docs.thinkwisesoftware.com/docs/kb/report_mailer.html#parameters 

it should be possible to send the attachment in for example .xls format?

in our task we added the following task parameter:

We have tried various different setting, so far no luck.

please advise,

thank you

Paul