When I asked the client if they have an existing API with xml or json, they said that they will send me Excel xlsx per email.

Parsing xlsx isn't too hard, why are you having issues?

/s

@ChlorideCull It's about an Rails web app. It would have been 1 line of code to serialise the data to json or xml.

Well in the end I created a view that would return a xls valid xml.

Now starting to write the parser for the returned data.

It's OK, they'll send you a new version if something changes and don't tell you what did.