注册时上传图片给用户时出错？

// 示例代码：前端上传图片
const fileInput = document.getElementById('fileInput');
const formData = new FormData();

fileInput.addEventListener('change', (event) => {
    const file = event.target.files[0];
    formData.append('file', file);

    fetch('/upload', {
        method: 'POST',
        body: formData
    })
    .then(response => response.json())
    .then(data => {
        console.log(data);
    })
    .catch(error => {
        console.error('Error:', error);
    });
});

# 示例代码：后端处理上传文件（Python Flask）
from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/upload', methods=['POST'])
def upload_file():
    if 'file' not in request.files:
        return jsonify({'error': 'No file part'}), 400
    file = request.files['file']
    if file.filename == '':
        return jsonify({'error': 'No selected file'}), 400
    if file:
        # 处理文件保存逻辑
        file.save('/path/to/save/' + file.filename)
        return jsonify({'message': 'File successfully uploaded'}), 200

if __name__ == '__main__':
    app.run(debug=True)

// 示例代码：前端文件类型和大小限制
fileInput.addEventListener('change', (event) => {
    const file = event.target.files[0];
    if (!file.type.startsWith('image/')) {
        alert('Only images are allowed');
        return;
    }
    if (file.size > 5 * 1024 * 1024) { // 5MB
        alert('File size exceeds limit');
        return;
    }
    // 继续上传逻辑
});