From testing, don't think this could exploited directly, as the response
would error instead of allowing control characters, but this adds an
extra layer of sanitization, and switches to encoded disposition
filenames for better UTF8 support.
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
