Add support for XHTML rendering

[noomorph]

Context

A bit exotic feature, but you never know when you need some obscure thing from 2000s. 😅

As a user of Allure reports, I have to attach XML files with embedded XSL stylesheets, and the least problematic MIME type for them is, surprisingly, application/xhtml+xml.

1. Browsers correctly render them when given URLs where the header contains Content-Type: application/xhtml+xml. Unfortunately, text/html is less predictable and there are rendering problems with it.
2. In Allure reports, *.xml files are rendered as a code by default, but we don't want this specifically for styled XML files - it is preferable if they render interactively, just like .html attachments. This MIME type serves as a clear intent that we are know what we are doing.

text/html (actual)	application/xhtml+xml (expected)

Checklist

• Sign Allure CLA

[noomorph]

Anything else to be done here?