Existing mime sniffer wasn't great at distinguishing between plaintext
file types, so added a custom extension based mapping for common web
formats that may be expected to be used with this.
Allows files to be placed within a "public" folder within a theme
directory which the contents of will served by BookStack for access.
- Only "web safe" content-types are provided.
- A static 1 day cache time it set on served files.
For #3904