Content-Disposition can parse BASE64 encoded filename

See gh-26463

spring-web/src/main/java/org/springframework/http/ContentDisposition.java

@@ -22,7 +22,10 @@ import java.nio.charset.StandardCharsets;
 import java.time.ZonedDateTime;
 import java.time.format.DateTimeParseException;
 import java.util.ArrayList;
+import java.util.Base64;
 import java.util.List;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -320,6 +323,7 @@ public final class ContentDisposition {
 		return new ContentDisposition("", null, null, null, null, null, null, null);
 	}
 
+	private final static Pattern BASE64_ENCODED_PATTERN = Pattern.compile("=\\?([0-9a-zA-Z-_]+)\\?B\\?([+/0-9a-zA-Z]+=*)\\?=");
 	/**
 	 * Parse a {@literal Content-Disposition} header value as defined in RFC 2183.
 	 * @param contentDisposition the {@literal Content-Disposition} header value
@@ -362,7 +366,14 @@ public final class ContentDisposition {
 					}
 				}
 				else if (attribute.equals("filename") && (filename == null)) {
-					filename = value;
+					final Matcher matcher = BASE64_ENCODED_PATTERN.matcher(value);
+					if (matcher.find()) {
+						String charsetName = matcher.group(1);
+						String encoded = matcher.group(2);
+						filename = new String(Base64.getDecoder().decode(encoded), Charset.forName(charsetName));
+					} else {
+						filename = value;
+					}
 				}
 				else if (attribute.equals("size") ) {
 					size = Long.parseLong(value);

spring-web/src/test/java/org/springframework/http/ContentDispositionTests.java

@@ -80,6 +80,18 @@ class ContentDispositionTests {
 						.build());
 	}
 
+	@Test
+	void parseBase64EncodedUTF8Filename() {
+		assertThat(parse("attachment; filename=\"=?UTF-8?B?5pel5pys6KqeLmNzdg==?=\"").getFilename())
+				.isEqualTo("日本語.csv");
+	}
+
+	@Test
+	void parseBase64EncodedShiftJISFilename() {
+		assertThat(parse("attachment; filename=\"=?SHIFT_JIS?B?k/qWe4zqLmNzdg==?=\"").getFilename())
+				.isEqualTo("日本語.csv");
+	}
+
 	@Test
 	void parseEncodedFilenameWithoutCharset() {
 		assertThat(parse("form-data; name=\"name\"; filename*=test.txt"))