Version 2.0 of the Excel Importer supports .xlsx files and has just been released in the appstore.
IF this makes it to the Excel related parts of Mendix PLEASE make the format an OPTION parameter. As in all praticality there will be an aweful lot of people stuck with the "older" XLS format for some time!!
It is true that apache releases a new version of the poi library already a few months ago.
However I don't think we will release a new excelimporter this year with the new xlsx format as supported extension. Since a long time this module is finally stable and performing quiet well.
The apache library that supports xslx is completly different from the library we are currently using so the full module has to be rebuild.
This will take a lot of time and a lot of test working before this can be released. We are going to release a new version with xlsx support and off course it will be configurable if you want to use xls or xlsx. But when and how this version is going to be released isn't certain
We imported poi v.3.6 into our project in order to write Java actions which used facilities that weren't in poi v.3.1, which shipped with that version of Mendix (18.104.22.168). We had to manipulate the project setup to make sure that it linked to the right version of poi, and that did stop us using the Mendix exporter in the modeller. We needed Java processing anyway because of the volumes and we're using reporting instead of Export to Excel, so these constraints are OK with our application; might be an approach for others.