base64转multipartfile文件

将Base64编码的字符串转换为MultipartFile文件，需要进行以下步骤：

1. 将Base64编码的字符串解码为字节数组。

2. 创建一个临时文件，将字节数组写入文件中。

3. 使用MultipartFile类的实现类，如CommonsMultipartFile，将临时文件转换为MultipartFile文件。

以下是示例代码：

import org.apache.commons.codec.binary.Base64;
import org.springframework.web.multipart.MultipartFile;
import org.springframework.web.multipart.commons.CommonsMultipartFile;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;

public class Base64ToMultipartFile {

    public static MultipartFile convert(String base64String) throws IOException {
        byte[] bytes = Base64.decodeBase64(base64String);

        File tempFile = File.createTempFile("temp", null);
        try (FileOutputStream fos = new FileOutputStream(tempFile)) {
            fos.write(bytes);
        }

        return new CommonsMultipartFile(tempFile);
    }
}

使用示例：

String base64String = "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