MP4 files not supported?

CKfinder 2.0 doesn't seem to handle .mp4 files.

I have included 'mp4' in the list of allowed extensions:

config.resourceType[3].allowedExtensions = 'swf,flv,mp4';

but when I upload an mp4 file, it takes several minutes to upload and then the uploaded file is 0kb.

However some mp4 files under 1gb have uploaded ok, so it may just affect mp4 files above 1gb. FLV files over 1gb also work ok, so there doesn't appear to be a file size restriction on the server.

Any ideas?