WSS : documents created in Office and saved in Nuxeo are all reacognized as Word file

Steps to reproduce :
1. Open Excel (or PowerPoint).
2. Create a document.
3. Save it in a Nuxeo workspace.
-> In Nuxeo, it is displayed with a word icon
-> the name of the document in Nuxeo has the .xls or .ppt extension
-> On the Summary tab, the file has no name (You have to rename it so that the file has a name).