首先在decidePolicyForNavigationResponse方法中打印出navigationResponse.response.suggestedFilename，可以看到文件名為=?UTF8?B?MjAyM+W5tOeOsOaVmeS4reW_g+aakeWBh+WAvOePreihqO+8iA==?= =?UTF8?B?N+aciDXml6UtOeaciDEy5pel77yJLmRvYw==?=

獲取文件名和文件大小的方法如下：

- (void)webView:(WKWebView *)webView decidePolicyForNavigationResponse:(WKNavigationResponse *)navigationResponse decisionHandler:(void (^)(WKNavigationResponsePolicy))decisionHandler {
        //獲取文件名稱
        NSString *fileName = navigationResponse.response.suggestedFilename;       
          
        if([fileName containString:@"=?UTF8?B?"]){

            NSString *encodedString = navigationResponse.response.suggestedFilename;

            NSString *decodedString = [self decodeComplexBase64FileName:encodedString];

            fileName = decodedString;
    }

        //獲取文件大小
        NSString *fileSize = [self getFileSize:navigationResponse.response.expectedContentLength];
                                
        if(self.fileSize.intValue == -1){

            self.fileSize = [self getFileSize:[NSData dataWithContentsOfURL:navigationResponse.response.URL].length];

        }                           
}

- (NSString *)decodeComplexBase64FileName:(NSString *)fileName {
    
    //URL安全處理
    NSString *safeBase64String = [fileName stringByReplacingOccurrencesOfString:@"-" withString:@"+"];
    safeBase64String = [safeBase64String stringByReplacingOccurrencesOfString:@"_" withString:@"/"];
    
    NSMutableString *decodedFileName = [NSMutableString string];
    
    NSArray *components = [safeBase64String componentsSeparatedByString:@" "];
    for (NSString *component in components) {
        if ([component hasPrefix:@"=?UTF8?B?"] && [component hasSuffix:@"?="]) {
            NSString *base64String = [component substringWithRange:NSMakeRange(9, component.length - 11)];
            NSData *decodedData = [[NSData alloc] initWithBase64EncodedString:base64String options:0];
            NSString *decodedComponent = [[NSString alloc] initWithData:decodedData encoding:NSUTF8StringEncoding];
            
            if (decodedComponent) {
                [decodedFileName appendString:decodedComponent];
            }
        } else {
            [decodedFileName appendString:component];
        }
    }
    
    return decodedFileName;
}