Somehow uploadprogress seems to be affecting a request unrelated. I am using Cyberduck on Windows to test WebDAV (over HTTPS). It successfully logs in, lists files, etc. But it does not download any files because it does not like that PROPFIND on a file gives a 405 status. Cut down log:

2014/06/27 23:10:50 [debug] 28436#0: *569278 http request line: "PROPFIND /video/avgn/t_screwattack_avgn_bugsbcc_901_gt.mp4 HTTP/1.1"
2014/06/27 23:10:50 [debug] 28436#0: *569278 http uri: "/video/avgn/t_screwattack_avgn_bugsbcc_901_gt.mp4"
2014/06/27 23:10:50 [debug] 28436#0: *569278 using configuration "\.mp4$"
2014/06/27 23:10:50 [debug] 28436#0: *569278 http finalize request: 405, "/video/avgn/t_screwattack_avgn_bugsbcc_901_gt.mp4?" a:1, c:1
2014/06/27 23:10:50 [debug] 28436#0: *569278 http special response: 405, "/video/avgn/t_screwattack_avgn_bugsbcc_901_gt.mp4?"
2014/06/27 23:10:50 [debug] 28436#0: *569278 http set discard body
2014/06/27 23:10:50 [debug] 28436#0: *569278 uploadprogress error-tracker error: 405
2014/06/27 23:10:50 [debug] 28436#0: *569278 uploadprogress error-tracker not tracking in this location
2014/06/27 23:10:50 [debug] 28436#0: *569278 HTTP/1.1 405 Not Allowed
Server: nginx
Date: Sat, 28 Jun 2014 06:10:50 GMT
Content-Type: text/html
Content-Length: 166
Connection: keep-alive
Keep-Alive: timeout=20

My configuration is like this (trying to keep only relevant parts):
server {
...

dav_methods off;
create_full_put_path on;
dav_access group:r all:r;
dav_ext_methods PROPFIND OPTIONS;

location ~ \.mp4$ {
mp4;
mp4_buffer_size 1m;
mp4_max_buffer_size 5m;
}
}

When I comment out the mp4 location the client downloads the file fine. But I do want to use the mp4 module to optimise for those files.