BB: uhttpd: fix HTTP incompatibilities in file handler

 * Fixes sending an extraneous message body for 204 and 304 resoponses which
   breaks Chrome in keep-alive mode.

 * Adds mimetypes for JSON and JSONP.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

Backport of r43078

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@43082 3c298f89-4303-0410-b956-a3cf2f4a3e73