[Mirrors] Incorrect mime-types for mozdev.bigsearcher.com

[Douglas E. Warner]

It appears that mozdev.bigsearcher.com is no longer serving XPI files 
as "application/x-xpinstall".  We would appreciate it if you could take a 
look into this.

-Doug

$ curl -I http://mozdev.bigsearcher.com/aboutconfigsearch/abnfigsearch.xpi
HTTP/1.1 200 OK
Date: Thu, 11 Oct 2007 13:55:23 GMT
[...]
Content-Type: text/plain
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