Support multiple base64 segments in Content-Disposition

See gh-28236

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

@@ -377,7 +377,13 @@ public final class ContentDisposition {
 						if (matcher.find()) {
 							charset = Charset.forName(matcher.group(1));
 							String encodedValue = matcher.group(2);
-							filename = new String(Base64.getDecoder().decode(encodedValue), charset);
+							StringBuilder sb = new StringBuilder(new String(Base64.getDecoder().decode(encodedValue), charset));
+							while (matcher.find()){
+								charset = Charset.forName(matcher.group(1));
+								encodedValue = matcher.group(2);
+								sb.append(new String(Base64.getDecoder().decode(encodedValue), charset));
+							}
+							filename = sb.toString();
 						}
 						else {
 							matcher = QUOTED_PRINTABLE_ENCODED_PATTERN.matcher(value);

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

@@ -86,6 +86,14 @@ class ContentDispositionTests {
 		assertThat(parse(input).getFilename()).isEqualTo("日本語.csv");
 	}
 
+	@Test
+	void parseBase64EncodedFilenameHasMoreSegments() {
+		/* https://datatracker.ietf.org/doc/html/rfc2047#section-2
+		* An 'encoded-word' may not be more than 75 characters long */
+		String input = "attachment; filename=\"=?utf-8?B?U3ByaW5n5qGG5p625Li65Z+65LqOSmF2YeeahOeOsOS7o+S8geS4muW6lA==?= =?utf-8?B?55So56iL5bqP5o+Q5L6b5LqG5YWo6Z2i55qE57yW56iL5ZKM6YWN572u5qih?= =?utf-8?B?5Z6LLnR4dA==?=\"";
+		assertThat(parse(input).getFilename()).isEqualTo("Spring框架为基于Java的现代企业应用程序提供了全面的编程和配置模型.txt");
+	}
+
 	@Test // gh-26463
 	void parseBase64EncodedShiftJISFilename() {
 		String input = "attachment; filename=\"=?SHIFT_JIS?B?k/qWe4zqLmNzdg==?=\"";