Send Test Mail page: typographical error

Page: /admin/SenTestMail.jspa

In the "Message Type" drop-down list, "Html" must be changed to
"HTML" (this is an initialism and has no meaning if not all
letters are capitalized).

An initialism is a series of capital letters, each of which is usually the initial of a word. If an initialism is pronouncable as a word, it is called an acronym.

Typographical errors convey the unwanted impression of carelessness.