Update HttpServer to detect mimetype of blog posts

While updating one of my projects to the latest Sculpin, I noticed that the built-in server was dishing out blog posts with a mimetype of "application/octet-stream". This made it difficult to preview my progress.

After tracing through the code I was able to narrow it down to the fact that my blog posts don't have an extension - nor to they exist as a folder with an index.html endpoint. They are just named files without extensions.

I used the Fileinfo extension (which appears to be included in >=5.3.0) to try and guess the mimetype of files without extensions. I didn't do much in the way of error checking, so hopefully this doesn't cause weird issues. :)