templates library: recognize xhtml extension

[ReubenM]

Chrome associates the extension as MIME type "application/xhtml+xml". (More strict rendering) The client doesn't show xhtml files when loading the server's template library. I'm assuming this is the server not the client that filters file extensions...

[ronag]

@pkeuter ping

[pkeuter]

What do we want with this? Currently the HTML producer only looks at .html files in the template directory, but since you want to eventually move away from the template directory, is this still relevant?

Referring to @Julusian in #797

It would solve the problem, but then we would instead be maintaining a list of file extensions, which would require updating (there is already one for html #314). I agree with including file extensions anyway, as a further clarification of what is loaded. What about case sensitivity? It would be worth considering now with linux support.

I am happy to include a file extension list for now, but since .xhtml and .htm files are not very common, and there is a workaround (PLAY 1-1 [HTML] file://C:\path\file.xhtml). I would vote for not implementing the file extension list, but rather for getting rid of playing without explicitly specifying an extension.

How do others think about this?