How to download file using Cowboy?

Question

I want to download file from the browser and I try to achieve by cowboy, however I failed and the browser show me that "Repeat header received from the server.". I have no idea, everyone please help me. this is my code of handler: `

%% @doc GET echo handler.
-module(toppage_handler2).

-export([init/3]).
-export([handle/2]).
-export([terminate/3]).

init(_Transport, Req, []) ->
    {ok, Req, undefined}.

handle(Req, State) ->
    {Method, Req2} = cowboy_req:method(Req),
    {Echo, Req3} = cowboy_req:qs_val(<<"echo">>, Req2),
    {ok, Req4} = echo(Method, <<Echo/binary, " I am there ">>, Req3),
    {ok, Req4, State}.

echo(<<"GET">>, undefined, Req) ->
    cowboy_req:reply(400, [], <<"Missing echo parameter.">>, Req);

%% the main part of download the file is here
%% I just want to download the file README.md
echo(<<"GET">>, Echo, Req) ->
    F = fun (Socket, Transport) ->
    Transport:sendfile(Socket, "priv/README.md")
    end,
    Req2 = cowboy_req:set_resp_body_fun(1024, F, Req),
     Req3 = cowboy_req:set_resp_header(<<"Content-Disposition">>, "GET", Req2),
    Req4 = cowboy_req:set_resp_header(<<"attachment;filename=\"README.md\"">>, "GET", Req3),
     Req5 = cowboy_req:set_resp_header(<<"Content-Length">>, "GET",  Req4),
     Req6 = cowboy_req:set_resp_header(<<"1024">>, "GET",  Req5),
    cowboy_req:reply(200, [
        {<<"content-type">>, <<"application/octet-stream">>}
    ], "", Req6);

echo(_, _, Req) ->
    %% Method not allowed.
    cowboy_req:reply(405, Req).

terminate(_Reason, _Req, _State) ->
    ok.`

Answer 1

Cowboy has a built in handler for serving static files. It is documented here:

http://ninenines.eu/docs/en/cowboy/HEAD/guide/static_handlers/

and there is example on github:

https://github.com/ninenines/cowboy/tree/master/examples/static_world/src

This way, you don't have to set headers manually, which should prevent the error.