无法将文件PDF上载到带有flask的文件夹

from flask import Flask, request
from werkzeug.utils import secure_filename

app = Flask(__name__)

# 设置允许上传的文件类型
ALLOWED_EXTENSIONS = {'pdf'}

# 设置上传文件的保存路径
UPLOAD_FOLDER = 'uploads'
app.config['UPLOAD_FOLDER'] = UPLOAD_FOLDER

# 检查文件扩展名是否允许上传
def allowed_file(filename):
    return '.' in filename and \
           filename.rsplit('.', 1)[1].lower() in ALLOWED_EXTENSIONS

@app.route('/upload', methods=['POST'])
def upload_file():
    # 检查是否有文件被上传
    if 'file' not in request.files:
        return 'No file uploaded', 400

    file = request.files['file']

    # 检查文件名是否为空
    if file.filename == '':
        return 'No selected file', 400

    # 检查文件类型是否允许上传
    if not allowed_file(file.filename):
        return 'Invalid file type', 400

    # 保存文件到指定的文件夹中
    filename = secure_filename(file.filename)
    file.save(os.path.join(app.config['UPLOAD_FOLDER'], filename))

    return 'File uploaded successfully'

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