下载的附件名总乱码？你该去读一下 RFC 文档了！

// MicroSoft Browser
if (agent.contains("msie") || agent.contains("trident") || agent.contains("edge")) {
  // filename 特殊处理
}
// firefox
else if (agent.contains("firefox")) {
  // filename 特殊处理
}
// safari
else if (agent.contains("safari")) {
  // filename 特殊处理
}
// Chrome
else if (agent.contains("chrome")) {
  // filename 特殊处理
}
// 其他
else{
 // filename 特殊处理
}
//最后把特殊处理后的文件名放到head里
response.setHeader("Content-Disposition",
                    "attachment;fileName=" + filename);

// percentEncodedFileName 为百分号编码后的文件名
response.setHeader("Content-disposition",
        "attachment;filename=" + percentEncodedFileName +
        ";filename*=utf-8''" + percentEncodedFileName);

content-disposition = "Content-Disposition" ":"
                            disposition-type *( ";" disposition-parm )
     disposition-type    = "inline" | "attachment" | disp-ext-type
                         ; case-insensitive
     disp-ext-type       = token
     disposition-parm    = filename-parm | disp-ext-parm
     filename-parm       = "filename" "=" value
                         | "filename*" "=" ext-value

Producers MUST use either the "UTF-8" ([RFC3629]) or the "ISO-8859-1"([ISO-8859-1]) character set.

Content-Disposition: attachment;
                      filename="EURO rates";
                      filename*=utf-8''%e2%82%ac%20rates

 A single quote is used to
   separate the character set, language, and actual value information in
   the parameter value string, and an percent sign is used to flag
   octets encoded in hexadecimal.

recipients implementing this specification
MUST support the character sets "ISO-8859-1" and "UTF-8".

A percent-encoding mechanism is used to represent a data octet in a
component when that octet's corresponding character is outside the
allowed set or is being used as a delimiter of, or within, the
component.  A percent-encoded octet is encoded as a character
triplet, consisting of the percent character "%" followed by the two
hexadecimal digits representing that octet's numeric value.  For
example, "%20" is the percent-encoding for the binary octet
"00100000" (ABNF: %x20), which in US-ASCII corresponds to the space
character (SP).  Section 2.4 describes when percent-encoding and
decoding is applied.

The default encoding for all forms is `application/x-www-form-
   urlencoded'. A form data set is represented in this media type as
   follows:
        1. The form field names and values are escaped: space
        characters are replaced by `+', and then reserved characters
        are escaped as per [URL]

@GetMapping("/downloadFile")
public String download(String serverFileName, HttpServletRequest request, HttpServletResponse response) throws IOException {
    request.setCharacterEncoding("utf-8");
    response.setContentType("application/octet-stream");
    String clientFileName = fileService.getClientFileName(serverFileName);
    // 对真实文件名进行百分号编码
    String percentEncodedFileName = URLEncoder.encode(clientFileName, "utf-8")
            .replaceAll("\\+", "%20");
    // 组装contentDisposition的值
    StringBuilder contentDispositionValue = new StringBuilder();
    contentDispositionValue.append("attachment; filename=")
            .append(percentEncodedFileName)
            .append(";")
            .append("filename*=")
            .append("utf-8''")
            .append(percentEncodedFileName);
    response.setHeader("Content-disposition",
            contentDispositionValue.toString());
    
    // 将文件流写到response中
    try (InputStream inputStream = fileService.getInputStream(serverFileName);
         OutputStream outputStream = response.getOutputStream()
    ) {
        IOUtils.copy(inputStream, outputStream);
    }
    return "OK!";
}