Fix NPEs in MimeMessageHelper when adding files without names

Prior to this commit, `MimeMessageHelper` would accept `Resource`
instances as inline attachments in multipart MIME messages. If the
provided `Resource` implementation returns `null` for `getFileName()`,
the `addInLine` method would fail with a `NullPointerException` as
Jakarta activation fails to detect the content type for a null filename.

This commit falls back on "application/octet-stream" when the filename
is not known for the given resource instead of failing with an
exception.

Fixes gh-33527

spring-context-support/src/main/java/org/springframework/mail/javamail/MimeMessageHelper.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -43,6 +43,7 @@
 import org.springframework.core.io.Resource;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.MimeTypeUtils;
 
 /**
  * Helper class for populating a {@link jakarta.mail.internet.MimeMessage}.
@@ -960,7 +961,8 @@ public void addInline(String contentId, File file) throws MessagingException {
 	 */
 	public void addInline(String contentId, Resource resource) throws MessagingException {
 		Assert.notNull(resource, "Resource must not be null");
-		String contentType = getFileTypeMap().getContentType(resource.getFilename());
+		String contentType = (resource.getFilename() != null ?
+				getFileTypeMap().getContentType(resource.getFilename()) : MimeTypeUtils.APPLICATION_OCTET_STREAM_VALUE);
 		addInline(contentId, resource, contentType);
 	}
 

spring-context-support/src/test/java/org/springframework/mail/javamail/JavaMailSenderTests.java

@@ -39,6 +39,7 @@
 import jakarta.mail.internet.MimeMessage;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.core.io.ByteArrayResource;
 import org.springframework.mail.MailParseException;
 import org.springframework.mail.MailSendException;
 import org.springframework.mail.SimpleMailMessage;
@@ -270,6 +271,25 @@ void javaMailSenderWithMimeMessageHelperAndDefaultEncoding() throws Exception {
 		assertThat(sender.transport.getSentMessages()).containsExactly(message.getMimeMessage());
 	}
 
+	@Test
+	void javaMailSenderWithMimeMessageHelperAndCustomResource() throws Exception {
+		sender.setHost("host");
+		sender.setUsername("username");
+		sender.setPassword("password");
+
+		MimeMessageHelper message = new MimeMessageHelper(sender.createMimeMessage(), true);
+		message.setTo("[email protected]");
+		message.addInline("id", new ByteArrayResource(new byte[] {1, 2, 3}));
+
+		sender.send(message.getMimeMessage());
+
+		assertThat(sender.transport.getConnectedHost()).isEqualTo("host");
+		assertThat(sender.transport.getConnectedUsername()).isEqualTo("username");
+		assertThat(sender.transport.getConnectedPassword()).isEqualTo("password");
+		assertThat(sender.transport.isCloseCalled()).isTrue();
+		assertThat(sender.transport.getSentMessages()).containsExactly(message.getMimeMessage());
+	}
+
 	@Test
 	void javaMailSenderWithParseExceptionOnSimpleMessage() {
 		SimpleMailMessage simpleMessage = new SimpleMailMessage();