1. 在Controller中加入以下代码段，将文件的大小和扩展名限制在特定范围内：

public async Task Upload(IFormFile file)
{
    if (file == null || file.Length == 0)
        return BadRequest("请选择文件！");

    var fileSizeLimit = 10000000; //文件大小限制为10MB
    var allowedExtensions = new[] { ".jpg", ".png", ".doc", ".pdf" }; //允许上传的扩展名
    var extension = Path.GetExtension(file.FileName);

    if (file.Length > fileSizeLimit)
        return BadRequest($"文件大小不能超过{fileSizeLimit / 1024 / 1024}MB");
    if (!allowedExtensions.Contains(extension.ToLower()))
        return BadRequest("不支持此文件类型");

    //保存文件并返回结果
}

2. 通过自定义验证器，在模型层面上验证上传的文件。首先，在Startup.cs中加入以下代码段：

services.Configure(options =>
{
    options.ModelValidatorProviders.Insert(0, new FileValidationProvider());
});

然后，新建FileValidationProvider.cs文件，实现自定义验证器：

public class FileValidationProvider : IModelValidatorProvider
{
    public void CreateValidators(ModelValidatorProviderContext context)
    {
        foreach (var modelMetadata in context.Results.Select(r => r.Metadata).Where(m => m.ModelType == typeof(IFormFile)))
        {
            context.Results.Add(new ValidatorItem { 
                Validator = new FileSizeValidator(modelMetadata),
                IsReusable = true });
            context.Results.Add(new ValidatorItem { 
                Validator = new FileTypeValidator(modelMetadata),
                IsReusable = true });
        }
    }

    private class ValidatorItem : ValidatorItemBase
    {
        public ValidatorItem() : base(string.Empty) { }
    }

    private class FileSizeValidator : AbstractValidator
    {
        private readonly int _maxSizeInBytes;

        public FileSizeValidator(ModelMetadata metadata)
            : base(metadata, new HttpContextAccessor())
        {
            _maxSizeInBytes = 10 * 1024 * 1024; //文件大小限制为10MB
        }

        public override string ErrorMessage { get; set; } = "文件大小不能超过10MB";

        public override bool IsValid(IFormFile value)
        {
            if (value == null || value.Length == 0)
                return true;

            return value.Length <= _maxSizeInBytes;
        }
    }

    private class FileTypeValidator : AbstractValidator
    {
        private readonly string[] _allowedExtensions;

        public FileTypeValidator(ModelMetadata metadata)
            : base