Before you go any further, have you successfully loaded a xls file in Mp3tag?
Adding an arbitrary extension to the options does not make Mp3tag able to work with this extension.
Ok, thanks. I will look more into the documentation. I am able to choose convert filename->filename and type ind the rules regex to {1}{2} etc. but can only press cancel since the "go" and "Example" buttons are greyed out.
I couldn't quite figure it out. Mainly because of a simple basic error:
it is not enough to list files in the windows. One also have to mark/highlight/choose them.
When that was done I could click the "Convert - Quick" and choose "With Regex"
and did:
field: _FILENAME
from: ([0-9]{2})([0-9]{2})([0-9]{4})
to: $3-$2-$1
All files named ddmmyyyy.xls are now yyyy-mm-dd.xls