decode_body: Improve tar detection

Detect when content-type is set to (arguably invalid)
`application/octet-stream; charset-utf8` and when there is none.

Closes #375

lib/req/steps.ex

@@ -1954,22 +1954,25 @@ defmodule Req.Steps do
   end
 
   defp format(request, response) do
+    # TODO: remove ` || ` when we require Elixir v1.13
+    path = request.url.path || ""
+
     case Req.Response.get_header(response, "content-type") do
       [content_type | _] ->
-        # TODO: remove ` || ` when we require Elixir v1.13
-        path = request.url.path || ""
-
         case extensions(content_type, path) do
           [ext | _] -> ext
           [] -> nil
         end
 
       [] ->
-        []
+        case extensions("application/octet-stream", path) do
+          [ext | _] -> ext
+          [] -> nil
+        end
     end
   end
 
-  defp extensions("application/octet-stream", path) do
+  defp extensions("application/octet-stream" <> _, path) do
     if tgz?(path) do
       ["tgz"]
     else

test/req/steps_test.exs

@@ -764,6 +764,28 @@ defmodule Req.StepsTest do
       assert Req.get!(plug: plug, url: "/foo.tar").body == files
     end
 
+    test "tar (path, content type with charset utf8)" do
+      files = [{~c"foo.txt", "bar"}]
+
+      plug = fn conn ->
+        conn
+        |> Plug.Conn.put_resp_content_type("application/octet-stream")
+        |> Plug.Conn.send_resp(200, create_tar(files))
+      end
+
+      assert Req.get!(plug: plug, url: "/foo.tar").body == files
+    end
+
+    test "tar (path, no content-type)" do
+      files = [{~c"foo.txt", "bar"}]
+
+      plug = fn conn ->
+        Plug.Conn.send_resp(conn, 200, create_tar(files))
+      end
+
+      assert Req.get!(plug: plug, url: "/foo.tar.gz").body == files
+    end
+
     test "tar.gz (path)" do
       files = [{~c"foo.txt", "bar"}]