in this link, it says
File handlers are a new type of Office add-in that integrate non-Microsoft file types into Office 365 in the same way that that Office file types are.
and 'docx' is Microsoft file type, so I think I can't make file handler for 'docx' format.
and I also did a simple test (that failed). but why I'm asking here is because I can't sure I did everything correctly.
Is it possible? whether it is or not, is it official state of Microsoft? are there any page links?
what I tested:
1. made a addin in azure active directory page.
2. in http: //addinsmanager.azurewebsites.net/, set its extension to docx and all urls to fake urls.
3. uploaded docx file to onedrive.
4. it still shows docx icon well, and word online is executed when I click the file.
5. I found "docx file" menu is added to new file drop down menu. when I click it, I'm sended to fake url. but, when I come back to onedrive and click the same file again, word online is executed. (instead of fake url)
Yes, you're correct that because DOCX is a Microsoft file type you can't make a File Handler add-in for it.
As the link you reference notes, File Handler add-ins can only add functionality for non-Microsoft files, such as CSV, MP4, DAT, EXE, RSS, RAR, or any other non-MS file extension you can think of.
-Michael (PM for Office add-ins)