[NiteWave]

my test:

before add
json application/json
into mime.properties,

#curl -I 127.0.0.1/a.json
HTTP/1.1 200 OK
Date: Fri, 07 Sep 2012 12:26:01 GMT
Server: LiteSpeed/4.1.13 Enterprise
Accept-Ranges: bytes
Connection: Keep-Alive
Keep-Alive: timeout=5, max=100
ETag: "27-5049e7af-f782c3"
Last-Modified: Fri, 07 Sep 2012 12:25:19 GMT
Content-Type: application/octet-stream
Content-Length: 39

after add,

#curl -I 127.0.0.1/a.json
HTTP/1.1 200 OK
Date: Fri, 07 Sep 2012 12:28:23 GMT
Server: LiteSpeed/4.1.13 Enterprise
Accept-Ranges: bytes
Connection: Keep-Alive
Keep-Alive: timeout=5, max=100
ETag: "27-5049e7af-f782c3"
Last-Modified: Fri, 07 Sep 2012 12:25:19 GMT
Content-Type: application/json
Content-Length: 39

so it's working as expected.

the server's mime setting may be overwritten in .htaccess or php script.

can you put an sampe a.json in your document root and do same tests as I did.