Observable using Firefox that respects mimetypes for CSS, mod_cache with mem_cache can fail to cache Content-Type, so that after initial passthrough, the default type is returned in the http headers. mod_disk does store Content-Type this in the store_headers call.
Please upgrade to httpd 2.2.x as mod_cache in 2.0.x is experimental and known to have many bugs that have been fixed in 2.2.x. *** This bug has been marked as a duplicate of 39266 ***