Elm:
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port module FileReader exposing (..)

-- Define a port to receive the file content as a String
port fileContent : (String -> msg) -> Sub msg

-- Define a message to signify file has been read
type Msg = FileRead String

-- Subscribe to the port in your app
subscriptions : Model -> Sub Msg
subscriptions model =
    fileContent FileRead

-- Update your model based on the message received
update : Msg -> Model -> (Model, Cmd Msg)
update (FileRead content) model =
    ({ model | fileContent = content }, Cmd.none)

// Assuming you have an Elm app initialized and running
const app = Elm.Main.init();

document.getElementById('file-upload').addEventListener('change', function(event) {
  const reader = new FileReader();
  
  reader.onload = function(event) {
    // Send the file content back to Elm
    app.ports.fileContent.send(event.target.result);
  };
  
  // Read the content of the chosen file as text
  reader.readAsText(event.target.files[0]);
});