curl --request GET \
  --url https://api.altostrat.io/radius/migration/{filename}/preview \
  --header 'Authorization: Bearer <token>'

{
  "data": {
    "filename": "rad_file_01HGN7RXYZABC123.csv",
    "preview": [
      [
        "<string>"
      ]
    ],
    "total_lines": 5001,
    "detected_columns": [
      {
        "id": "col_0",
        "name": "username",
        "sample": "test@example.com"
      }
    ],
    "delimiter": "comma",
    "skipped_lines": 1
  }
}