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.InputStreamSource;
 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 class MimeMessageHelper {
 	 */
 	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);
 	}