Cant convert “application/x-www-form-urlencoded” to “application/json” JavaScript Flask

I’m trying to pull image from client(javascript) to sever(flask) but i got error while doing that. Even i tried to set ‘Content-Type’: ‘application/json’ but when i return the ‘content_type’ it gave me ‘application/x-www-form-urlencoded’. Please help me.

This is my app.py

    @app.route('/')
    @app.route('/index')
    def index():
        return render_template('index.html')

    @app.route('/save-image', methods=['POST'])
    def save_image():

        content_type = request.headers.get('Content-Type')
        if (content_type != 'application/json'):
            return content_type
        # This return application/x-www-form-urlencoded

        data = request.get_json()
        if not data or not data.get('imageData'):
            return 'Missing image data'

        image_data = data['imageData']

        image_data = image_data.split(',')[1]
        image_data = base64.b64decode(image_data)
        filename = f'{uuid.uuid4()}.png'
        filepath = os.path.join('images', filename)

This is my JavaScript

<button id="snap">Take Photo</button>
<form method="POST" action="/save-image">
  <video id="video" width="600" height="440" autoplay style="background-color: grey"></video>
  <canvas name="image" id="canvas" width="600" height="440" style="background-color: grey"></canvas>
  <button onclick="sendCanvasImage()">Convert to Image</button>
</form>

<script>
// Elements for taking the snapshot
var video = document.getElementById('video');
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');

// Get access to the camera!
if(navigator.mediaDevices && navigator.mediaDevices.getUserMedia) {
  navigator.mediaDevices.getUserMedia({ video: true })
    .then(function(stream) {
      video.srcObject = stream;
      video.play();
    });
}

document.getElementById("snap").addEventListener("click", function() {
  context.drawImage(video, 0, 0, 600, 400);
});

async function sendCanvasImage() {
  var canvas = document.getElementById('canvas');
  var imageData = canvas.toDataURL('image/png'); // Specify image format (optional)

  var response = await fetch('/save-image', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json' // Set content type
    },
    body: JSON.stringify({ imageData }) // Send data in JSON format
  });
}
</script>