使用erlang编写一个http接口处理代码，方法为post、get、delete，监听端口为12500，post仅能够上传JPG类型的文件，上传其余类型报错500；get能够通过文件名获取文件，文件不存在返回报错404；delete可以删除上传的文件

以下是使用 Erlang 编写的一个简单的 HTTP 服务器，可以处理指定要求：

-module(http_server).
-behaviour(gen_server).

-export([start/0, stop/0]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2, code_change/3]).
-export([start_listener/1, accept/1]).

start() ->
    gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).

stop() ->
    gen_server:cast(?MODULE, stop).

init([]) ->
    {ok, Listen} = gen_tcp:listen(12500, [{active, false}, {reuseaddr, true}]),
    {ok, #state{listen_socket = Listen}}.

handle_call(_Request, _From, State) ->
    {reply, ok, State}.

handle_cast(_Msg, State) ->
    {noreply, State}.

handle_info({tcp, Socket, Data}, State) ->
    spawn(fun() -> handle_request(Socket, Data) end),
    {noreply, State};

handle_info(stop, State) ->
    {stop, normal, ok, State};

handle_info(_, State) ->
    {noreply, State}.

terminate(_Reason, _State) ->
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

start_listener(Listen) ->
    {ok, Socket} = gen_tcp:accept(Listen),
    gen_tcp:controlling_process(Socket, self()),
    accept(Socket).

accept(Socket) ->
    case gen_tcp:recv(Socket, 0) of
        {ok, Data} ->
            spawn(fun() -> handle_request(Socket, Data) end),
            accept(Socket);
        {error, _} ->
            ok
    end.

handle_request(Socket, Data) ->
    case http_parser:parse_request(Data) of
        {ok, Request} ->
            case Request#http_request.method of
                'GET' ->
                    handle_get_request(Socket, Request);
                'POST' ->
                    handle_post_request(Socket, Request);
                'DELETE' ->
                    handle_delete_request(Socket, Request);
                _ ->
                    send_error_response(Socket, 400, "Bad Request", "Invalid method")
            end;
        {error, Reason} ->
            send_error_response(Socket, 400, "Bad Request", Reason)
    end,
    gen_tcp:close(Socket).

handle_get_request(Socket, Request) ->
    Filepath = Request#http_request.uri,
    case file:read_file(Filepath) of
        {ok, Data} ->
            send_response(Socket, 200, "OK", Data);
        {error, enoent} ->
            send_error_response(Socket, 404, "Not Found", "File not found");
        _ ->
            send_error_response(Socket, 500, "Internal Server Error", "Failed to read file")
    end.

handle_post_request(Socket, Request) ->
    {_, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, _, Headers, Body} = Request,
    case proplists:get_value("content-type", Headers) of
        "image/jpeg" ->
            case file:write_file("file.jpg", Body) of
                ok ->
                    send_response(Socket, 201, "Created", "");
                _ ->
                    send_error_response(Socket, 500, "Internal Server Error", "Failed to write file")
            end;
        _ ->
            send_error_response(Socket, 500, "Internal Server Error", "Invalid file type")
    end.

handle_delete_request(Socket, Request) ->
    Filepath = Request#http_request.uri,
    case file:delete(Filepath) of
        ok ->
            send_response(Socket, 204, "No Content", "");
        _ ->
            send_error_response(Socket, 500, "Internal Server Error", "Failed to delete file")
    end.

send_response(Socket, Status, Reason, Body) ->
    Response = http_parser:format_response(#http_response{status = Status, reason = Reason}, Body),
    gen_tcp:send(Socket, Response).

send_error_response(Socket, Status, Reason, Body) ->
    Response = http_parser:format_response(#http_response{status = Status, reason = Reason}, Body),
    gen_tcp:send(Socket, Response).

以上代码实现了一个简单的 HTTP 服务器，可以通过 GET 方法获取文件内容，通过 POST 方法上传 JPG 文件，通过 DELETE 方法删除文件。其中，GET 方法会返回 404 错误，POST 方法会返回 500 错误，DELETE 方法会返回 500 错误，如果文件不存在或文件类型不正确的话。