.mht is a filetype extention for MHTML Documents.
What are they?
.mht files are archived web pages. People using non-free software make these files by saving web pages using propietary software.
What to do if you get a .mht file
Delete it and politely educate the obviously mind-controlled non-thinking non-free software using drone who provided you with this file to provide you with the content using a free and open format.
How to view .mht files using free software
It is not possible to view .mht files using free software. It is, however, possible to open them using the non-free (as in freedom, not price) Opera web brwoser on GNU/Linux systems.