When trying to upload a .docx file, the file manager module reports:

The File C:\DNN\Portals\0\HOLD\200904helper.docx Is A Restricted File Type. Valid File Types Include ( *. swf, *.jpg, *.jpeg, *.jpe, *.gif, *.bmp, *.png, *.doc, *.xls, *.ppt, *.pdf, *.txt, *.xml, *.xsl, *.css, *.zip, *.template ). Please Contact Your Hosting Provider If You Need To Upload A File Type Which Is Not Supported.

The IIS 6 webserver is configured for all MIME types including .docx (application/vnd.ms-word.document.12) .

How can DNN File Manager be configured for more MIME types?

This hasn't been changed in DNN 5.