If you receive a message that looks similar to this:

The File C:\Inetpub\XXXXXXX\httpdocs\Portals\0\filename.mht Is A Restricted File Type. Valid File Types Include ( *. swf, *.jpg, *.jpeg, *.jpe, *.gif, *.bmp, *.png, *.doc, *.docx, *.xls, *.xlsx, *.ppt, *.pptx, *.pdf, *.txt, *.xml, *.xsl, *.xsd, *.css, *.zip, *.template, *.csv, *.htmtemplate ). Please Contact Your Hosting Provider If You Need To Upload A File Type Which Is Not Supported.

You will need to allow the MHT or MHTL file types as allowable extensions in the HOST / HOST SETTINGS / OTHER. Area of your website. Add ",mht,mhtml" (without the "") into the Allowable File Extensions area, and click update.