https://help.aliyun.com/knowledge_detail/39522.html?spm=5176.11065259.1996646101.searchclickresult.124c5f2fmoEsDJ

 /**
     * 通过文件名判断并获取OSS服务文件上传时文件的contentType
     *
     * @param fileName 文件名
     * @return 文件的contentType
     */
    public static final String getContentType(String fileName) {
        String FilenameExtension = fileName.substring(fileName.lastIndexOf("."));
        if (FilenameExtension.equalsIgnoreCase(".bmp")) {
            return "application/x-bmp";
        }
        if (FilenameExtension.equalsIgnoreCase(".gif")) {
            return "image/gif";
        }
        if (FilenameExtension.equalsIgnoreCase(".jpeg") ||
                FilenameExtension.equalsIgnoreCase(".jpg") ||
                FilenameExtension.equalsIgnoreCase(".png")) {
            return "image/jpeg";
        }
        if (FilenameExtension.equalsIgnoreCase(".html")) {
            return "text/html";
        }
        if (FilenameExtension.equalsIgnoreCase(".txt")) {
            return "text/plain";
        }
        if (FilenameExtension.equalsIgnoreCase(".vsd")) {
            return "application/vnd.visio";
        }
        if (FilenameExtension.equalsIgnoreCase(".pptx") ||
                FilenameExtension.equalsIgnoreCase(".ppt")) {
            return "application/vnd.ms-powerpoint";
        }
        if (FilenameExtension.equalsIgnoreCase(".docx") ||
                FilenameExtension.equalsIgnoreCase(".doc")) {
            return "application/msword";
        }
        if (FilenameExtension.equalsIgnoreCase(".xla") ||
                FilenameExtension.equalsIgnoreCase(".xlc")||
                    FilenameExtension.equalsIgnoreCase(".xlm")||
                        FilenameExtension.equalsIgnoreCase(".xls")||
                            FilenameExtension.equalsIgnoreCase(".xlt")||
                                FilenameExtension.equalsIgnoreCase(".xlw")) {
            return "application/vnd.ms-excel";
        }
        if (FilenameExtension.equalsIgnoreCase(".xml")) {
            return "text/xml";
        }
        if (FilenameExtension.equalsIgnoreCase(".pdf")) {
            return "application/pdf";
        }
        if (FilenameExtension.equalsIgnoreCase(".zip")) {
            return "application/zip";
        }
        if (FilenameExtension.equalsIgnoreCase(".tar")) {
            return "application/x-tar";
        }
        if (FilenameExtension.equalsIgnoreCase(".avi")) {
            return "video/avi";
        }
        if (FilenameExtension.equalsIgnoreCase(".mp4")) {
            return "video/mpeg4";
        }
        if (FilenameExtension.equalsIgnoreCase(".mp3")) {
            return "audio/mp3";
        }
        if (FilenameExtension.equalsIgnoreCase(".mp2")) {
            return "audio/mp2";
        }
        return "application/octet-stream";
    }