Maybe I jumped a bit too quick to a conclusion. I can confirm that it does use the "W" icon for .docx files.
However, the "X" icon is not used for .xlsx files, just as the "P" icon is not used for .pptx and .ppsx files. I'm not sure how the others are.
The MIME-type of the .xlsx file is "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", the MIME-type of the .pptx file is "application/vnd.openxmlformats-officedocument.presentationml.presentation", the MIME-type of the .ppsx file is "application/vnd.openxmlformats-officedocument.presentationml.slideshow".
Maybe I jumped a bit too quick to a conclusion. I can confirm that it does use the "W" icon for .docx files.
However, the "X" icon is not used for .xlsx files, just as the "P" icon is not used for .pptx and .ppsx files. I'm not sure how the others are.
The MIME-type of the .xlsx file is "application/ vnd.openxmlform ats-officedocum ent.spreadsheet ml.sheet" , the MIME-type of the .pptx file is "application/ vnd.openxmlform ats-officedocum ent.presentatio nml.presentatio n", the MIME-type of the .ppsx file is "application/ vnd.openxmlform ats-officedocum ent.presentatio nml.slideshow" .