.MHTM File

The mhtm file extension is one of the two file suffixes used for MHTML. 

MHTML, short for MIME HTML, is a web page archive format used to bind resources which are typically represented by external links (such as images, Flash animations, Java applets, audio files) together with HTML code into a single file. The first of the file is normally encoded HTML, subsequent parts are additional resources identified by their original URLs and encoded in base64. This format is sometimes referred to as MHT, after the suffix .mht given to such files by default when created by Microsoft Word, Internet Explorer, Opera or Firefox.

MIME types:
multipart/related
message/rfc822

How to open:

Use Microsoft Office Word 2002 or higher or Internet Explorer, Firefox, Opera.

How to convert:

Probably cannot be converted to other formats.