Case with NotAcceptableError while format is described

Hello guys,
I have weird problem with :accepts inside my plug. My controller action is plugged to the pipeline below.

  pipeline :browser do
    plug(:accepts, ["html", "image"])
  end

Inside my config file i have mime described as: config :mime, :types, %{"image/*" => ["image"]}
And inside my controller I have this code:

    file_path = Web.Env.path("priv/static/img/pixel.png")
    send_download(conn, {:file, file_path})

And when I’m testing this on my local environment or even prod environment everything works fine. But there are some cases, which I don’t know why happens. When function with send_download returns:

Phoenix.NotAcceptableError

no supported media type in accept header.

Expected one of ["html", "image"] but got the following formats:

  * "image/webp" with extensions: ["webp"]
  "image/png" with extensions: ["png"]
  "image/jpeg" with extensions: ["jpg", "jpeg", "jpe", "jfif"]
  "image/gif" with extensions: ["gif"]

To accept custom formats, register them under the :mime library
in your config/config.exs file:

    config :mime, :types, %{
      "application/xml" => ["xml"]
    }

And then run `mix deps.clean --build mime` to force it to be recompiled.

Can someone explain me what am I doing wrong? Why does it only sometimes doesn’t work?