IMCE Mkdir is a remote file upload vulnerablity on durpal platform,
normaly you can upload .txt extentions on websites
but some sites allowes you to upload .html files
if you want to upload shell on website then try in .phtml extention

to acess your shell/deface/file go here 
http://www.website.com/abc/files/abc/yourfilehere 

(replace abc with directory of website)

Leave comment if any query :) stay connected for More !

Live demo

Result

Other Demos 

http://correaporto.com.br/english/imce?dir=.

http://www.somaly.org/imce?dir=