Minimalistic example that exposes unexpected file:sendfile/5 behaviour.

sendfile.erl

-module(sendfile).

%API
-export([server/0, clients/0]).

%Internal loops
-export([client/1, acceptor/2, loop/2]).


server() ->
    {ok, Socket} = gen_tcp:listen(8008, [{ip, {127, 0, 0, 1}}, {active, false}]),
    [spawn_link(?MODULE, acceptor, [Socket, N]) || N <- lists:seq(1, 10)],
    ok.

clients() ->
    [spawn_link(?MODULE, client, [N]) || N <- lists:seq(1, 50)],
    ok.

acceptor(ListenSocket, N) ->
    case gen_tcp:accept(ListenSocket, 60000) of
        {ok, Socket} ->
            ?MODULE:loop(Socket, N),
            ?MODULE:acceptor(ListenSocket, N);
        _ -> ok
    end.

loop(Socket, N) ->
    case receive_data(Socket) of
        closed ->
            ok;
        Path ->
            sendfile(Socket, Path),
            io:format("acceptor~p has send a file.~n", [N]),
            %timer:sleep(100),
            ?MODULE:loop(Socket, N)
    end.

client(N) ->
    {ok, Socket} = gen_tcp:connect({127, 0, 0, 1}, 8008, [{active, false}]),
    send_path(Socket),
    case receive_data(Socket) of
        closed ->
            ok;
        File ->
            io:format("client~p has received ~p bytes.~n", [N, length(File)])
    end,
    gen_tcp:close(Socket).

receive_data(Socket) ->
    case gen_tcp:recv(Socket, 0) of
        {ok, Data} ->
            Data;
        {error, closed} ->
            closed;
        Error ->
            gen_tcp:close(Socket),
            exit(Error)
    end.

sendfile(Socket, Path) ->
    {ok, Fd} = file:open(Path, [read, raw, binary]),
    file:sendfile(Fd, Socket, 0, 0, []),
    file:close(Fd).

sendfile2(Socket, Path) ->
    {ok, File} = file:read_file(Path),
    gen_tcp:send(Socket, File).

send_path(Socket) ->
    gen_tcp:send(Socket, ?FILE).