diff --git a/graphql-cohttp/src/graphql_cohttp.ml b/graphql-cohttp/src/graphql_cohttp.ml
index cb5139c..0f2e6ea 100644
--- a/graphql-cohttp/src/graphql_cohttp.ml
+++ b/graphql-cohttp/src/graphql_cohttp.ml
@@ -1,10 +1,8 @@
 module type HttpBody = sig
   type t
-
   type +'a io
 
   val to_string : t -> string io
-
   val of_string : string -> t
 end
 
@@ -12,7 +10,6 @@ module type S = sig
   module IO : Cohttp.S.IO
 
   type body
-
   type 'ctx schema
 
   type response_action =
@@ -31,9 +28,7 @@ end
 
 module Option = struct
   let bind t ~f = match t with None -> None | Some x -> f x
-
   let map t ~f = bind t ~f:(fun x -> Some (f x))
-
   let first_some t t' = match t with None -> t' | Some _ -> t
 end
 
@@ -101,8 +96,8 @@ module Params = struct
       | Some query ->
           Ok
             ( query,
-              ( params.variables
-                :> (string * Graphql_parser.const_value) list option ),
+              (params.variables
+                :> (string * Graphql_parser.const_value) list option),
               params.operation_name )
       | None -> Error "Must provide query string"
     with Yojson.Json_error msg -> Error msg
@@ -125,13 +120,14 @@ struct
   type 'conn callback =
     'conn -> Cohttp.Request.t -> Body.t -> response_action Io.t
 
-  let respond_string ~status ~body () =
+  let respond_string ?headers ~status ~body () =
+    let headers = Option.map ~f:Cohttp.Header.of_list headers in
     Io.return
-      (`Response (Cohttp.Response.make ~status (), Body.of_string body))
+      (`Response (Cohttp.Response.make ?headers ~status (), Body.of_string body))
 
-  let static_file_response path =
+  let static_file_response ?headers path =
     match Assets.read path with
-    | Some body -> respond_string ~status:`OK ~body ()
+    | Some body -> respond_string ?headers ~status:`OK ~body ()
     | None -> respond_string ~status:`Not_found ~body:"" ()
 
   let execute_query ctx schema variables operation_name query =
@@ -156,7 +152,7 @@ struct
             respond_string ~status:`Bad_request ~body ()
         | Error err ->
             let body = Yojson.Basic.to_string err in
-            respond_string ~status:`Bad_request ~body () )
+            respond_string ~status:`Bad_request ~body ())
 
   let make_callback :
       (Cohttp.Request.t -> 'ctx) -> 'ctx Schema.schema -> 'conn callback =
@@ -170,15 +166,17 @@ struct
       | Some s -> List.mem "text/html" (String.split_on_char ',' s)
     in
     match (req.meth, path_parts, accept_html) with
-    | `GET, [ "graphql" ], true -> static_file_response "index.html"
+    | `GET, [ "graphql" ], true ->
+        static_file_response
+          ~headers:[ ("Content-Type", "text/html") ]
+          "index.html"
     | `GET, [ "graphql" ], false ->
         if
           Cohttp.Header.get headers "Connection" = Some "Upgrade"
           && Cohttp.Header.get headers "Upgrade" = Some "websocket"
         then
           let handle_conn =
-            Websocket_transport.handle
-              (execute_query (make_context req) schema)
+            Websocket_transport.handle (execute_query (make_context req) schema)
           in
           Io.return (Ws.upgrade_connection req handle_conn)
         else execute_request schema (make_context req) req body