i think your data-format .xlsx is Content of your Flowfile and you don´t have to convert it firtstly?
So you can easy add a UpdateAttribute Processor and add the Dynamic-Property "filename"
With this Property you overwrite the actually set Filename (you can see there is already an Attribute called "filename" from beginning).
As Value you can set the name via Attribute and add the format there
or just hard coded like
And after that you add a PutFile or FTP or whatever Processor and send it to the Path you set there.