Mime Settings for Unity3D file type

Most servers should require no configuration at all. Just upload the .unity3d file, and the accompanying html file, and you're finished.

Sometimes you'll get a server which is a bit particular about what kinds of files it serves up (looking at you, IIS!) so you need to explicitly add the unity webplayer mime type before it will even admit that your file exists to the outside world.

The MIME type for Unity webplayer content is:

application/vnd.unity

And the file extension for unity webplayer files is:

.unity3d

So, if you're editing htaccess files on your server, you need to add this:

AddType application/vnd.unity unity3d

And if you're configuring a server via some other means (eg, a control panel, or asking your sysadmin to do it) you just need to specify the mime type mentioned above.

(note, this question has already been asked and answered here but it looks as though the original asker will never return to accept the answer, so I'm reposting the answer in full here).