* [Mail/Attachments/Usability] Addressed an issue where malformed HTML attachments were being truncated in some situations if displayed in the browser. This was caused by the HTML Purifier code introduced in 6.2.5, which removes any potentially malicious Javascript in an HTML attachment that would be executed by a worker within an active Cerb session.  This code relied on PHP's DOMDocument, which is intolerant of malformed HTML markup.  The code will now take advantage of the Tidy extension if it's loaded.  This repairs malformed markup before DOMDocument loads it, HTML Purifier cleans it, and Cerb displays it.
* [Mail/Attachments/Usability] When hovering over a file attachment with the mouse, a 'download' link is now revealed.  Clicking this link will prompt for an attachment to be downloaded rather than displayed in the browser.  Previously, clicking on an attachment always attempted to display it in the browser first (e.g. images, sound files, videos, PDFs, HTML, etc).  When displaying HTML content in the browser, Cerb repairs and purifies it to remove Javascript.  When downloading an HTML attachment, the raw file is provided without any sanitization.  This is useful behavior if you're sharing HTML files with safe contacts and you do not want the contents to be modified.  By downloading HTML, any potentially malicious Javascript no longer has access to the worker's Cerb session.
Merge branch '6.3.2' into stable