Improve Extensions for Report Attachment

[ghostcity]

Dear Damian, in many cases the HTML report does not specify a correct attachment 's extension, even if it could be done.

For example, for Mime-Types with additional encoding (e.g., "text/html; charset=UTF-8") the extension is "unknown" instead of "html". Without a specific encoding, "html" extension is used.

For further example, the subtype of an unknown Mime-Type (e.g., "text/php") might be a good hint to deduce the extension.

Even further, the name itself could be interpreted as a file-name and, thus, the given extension could be used.

[damianszczepanik]

Prepare PR with changes so we can review then and discuss before merge