The data race detector reports:

WARNING: DATA RACE at 0x000040339450
Write by goroutine 15:
  os.(*file).close()
      src/pkg/os/file_unix.go:110 +0xf4
  os.(*File).Close()
      src/pkg/os/file_unix.go:99 +0x3a
  net/http_test.TestServeContent()
      src/pkg/net/http/fs_test.go:626 +0x16c4
  testing.tRunner()
      src/pkg/testing/testing.go:301 +0x86

Previous read by goroutine 142:
  net.sendFile()
      src/pkg/net/sendfile_linux.go:47 +0x5ce
  net.(*TCPConn).ReadFrom()
      src/pkg/net/tcpsock_posix.go:80 +0x40
  net/http.(*response).ReadFrom()
      src/pkg/net/http/server.go:161 +0x113
  io.CopyN()
      src/pkg/io/io.go:300 +0x154
  net/http.serveContent()
      src/pkg/net/http/fs.go:236 +0xa92
  net/http.ServeContent()
      src/pkg/net/http/fs.go:118 +0x17b
  net/http_test.func·024()
      src/pkg/net/http/fs_test.go:551 +0x193
  net/http.HandlerFunc.ServeHTTP()
      src/pkg/net/http/server.go:729 +0x3f
  net/http/httptest.(*waitGroupHandler).ServeHTTP()
      src/pkg/net/http/httptest/server.go:180 +0xc2
  net/http.(*conn).serve()
      src/pkg/net/http/server.go:681 +0x787

Goroutine 15 (running) created at:
  testing.RunTests()
      src/pkg/testing/testing.go:377 +0x974
  testing.Main()
      src/pkg/testing/testing.go:313 +0xad
  main.main()
      net/http/_test/_testmain.go:301 +0x95
  runtime.main()
      src/pkg/runtime/proc.c:248 +0x91

Goroutine 142 (running) created at:
  net/http.(*Server).Serve()
      src/pkg/net/http/server.go:1116 +0x444

The report looks false. The test uses syscall.SendFile() for synchronization.
Need to teach race detector to understand that.