要在Android中发送带附件的电子邮件，可以使用JavaMail库。下面是一个示例代码：

1. 首先，确保在build.gradle文件中添加以下依赖项：

implementation 'com.sun.mail:android-mail:1.6.2'
implementation 'com.sun.mail:android-activation:1.6.2'

2. 创建一个帮助类用于发送电子邮件。例如，创建一个名为EmailHelper的类。

import android.os.AsyncTask;
import java.util.Properties;
import javax.activation.DataHandler;
import javax.activation.DataSource;
import javax.activation.FileDataSource;
import javax.mail.BodyPart;
import javax.mail.Message;
import javax.mail.MessagingException;
import javax.mail.Multipart;
import javax.mail.PasswordAuthentication;
import javax.mail.Session;
import javax.mail.Transport;
import javax.mail.internet.InternetAddress;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;

public class EmailHelper extends AsyncTask {
    private String senderEmail;
    private String senderPassword;
    private String recipientEmail;
    private String subject;
    private String message;
    private String attachmentFilePath;

    public EmailHelper(String senderEmail, String senderPassword, String recipientEmail, String subject, String message, String attachmentFilePath) {
        this.senderEmail = senderEmail;
        this.senderPassword = senderPassword;
        this.recipientEmail = recipientEmail;
        this.subject = subject;
        this.message = message;
        this.attachmentFilePath = attachmentFilePath;
    }

    @Override
    protected Void doInBackground(Void... params) {
        Properties props = new Properties();
        props.put("mail.smtp.auth", "true");
        props.put("mail.smtp.starttls.enable", "true");
        props.put("mail.smtp.host", "smtp.gmail.com");
        props.put("mail.smtp.port", "587");

        Session session = Session.getInstance(props,
                new javax.mail.Authenticator() {
                    protected PasswordAuthentication getPasswordAuthentication() {
                        return new PasswordAuthentication(senderEmail, senderPassword);
                    }
                });

        try {
            MimeMessage mimeMessage = new MimeMessage(session);
            mimeMessage.setFrom(new InternetAddress(senderEmail));
            mimeMessage.addRecipient(Message.RecipientType.TO, new InternetAddress(recipientEmail));
            mimeMessage.setSubject(subject);

            Multipart multipart = new MimeMultipart();

            // 添加文本内容
            BodyPart messageBodyPart = new MimeBodyPart();
            messageBodyPart.setText(message);
            multipart.addBodyPart(messageBodyPart);

            // 添加附件
            if (attachmentFilePath != null && !attachmentFilePath.isEmpty()) {
                BodyPart attachmentBodyPart = new MimeBodyPart();
                DataSource source = new FileDataSource(attachmentFilePath);
                attachmentBodyPart.setDataHandler(new DataHandler(source));
                attachmentBodyPart.setFileName(attachmentFilePath);
                multipart.addBodyPart(attachmentBodyPart);
            }

            mimeMessage.setContent(multipart);

            Transport.send(mimeMessage);
        } catch (MessagingException e) {
            e.printStackTrace();
        }
        return null;
    }
}

3. 在需要发送邮件的Activity或Fragment中，可以使用以下代码调用EmailHelper类。

String senderEmail = "your_email@gmail.com";
String senderPassword = "your_password";
String recipientEmail = "recipient_email@gmail.com";
String subject = "Email Subject";
String message = "Email Message";
String attachmentFilePath = "/path/to/attachment/file.pdf";

EmailHelper emailHelper = new EmailHelper(senderEmail, senderPassword, recipientEmail, subject, message, attachmentFilePath);
emailHelper.execute();

请确保将上述代码中的“your_email@gmail.com”替换为实际的发件人电子邮件地址，“your_password”替换为实际的发件人电子邮件密码，“recipient_email@gmail.com”替换为实际的收件人电子邮件地址，以及“/path/to/attachment/file.pdf”替换为实际的附件文件路径。

这样，当调用EmailHelper的execute方法时，它将在后台发送电子邮件。