fix: don't warn on .text() usage with image/svg+xml (#6269)

Author: Cherry

src/workerd/util/mimetype-test.c++

@@ -387,6 +387,27 @@ KJ_TEST("WHATWG tests") {
   KJ_ASSERT(MimeType::isText(MimeType::XJAVASCRIPT));
   KJ_ASSERT(MimeType::isText(
       KJ_ASSERT_NONNULL(MimeType::tryParse("application/json; charset=\"utf-8\""_kj))));
+
+  auto svgMime = KJ_ASSERT_NONNULL(MimeType::tryParse("image/svg+xml"_kj));
+  KJ_ASSERT(MimeType::isXml(svgMime));
+  KJ_ASSERT(MimeType::isText(svgMime));
+
+  auto svgMimeWithCharset =
+      KJ_ASSERT_NONNULL(MimeType::tryParse("image/svg+xml; charset=utf-8"_kj));
+  KJ_ASSERT(MimeType::isXml(svgMimeWithCharset));
+  KJ_ASSERT(MimeType::isText(svgMimeWithCharset));
+
+  KJ_ASSERT(MimeType::isXml(MimeType::XHTML));
+
+  auto atomXml = KJ_ASSERT_NONNULL(MimeType::tryParse("application/atom+xml"_kj));
+  KJ_ASSERT(MimeType::isXml(atomXml));
+
+  auto textXml = KJ_ASSERT_NONNULL(MimeType::tryParse("text/xml"_kj));
+  KJ_ASSERT(MimeType::isXml(textXml));
+  auto appXml = KJ_ASSERT_NONNULL(MimeType::tryParse("application/xml"_kj));
+  KJ_ASSERT(MimeType::isXml(appXml));
+  auto imageXml = KJ_ASSERT_NONNULL(MimeType::tryParse("image/xml"_kj));
+  KJ_ASSERT(!MimeType::isXml(imageXml));
 }
 
 KJ_TEST("Extract Mime Type") {

src/workerd/util/mimetype.h

@@ -113,8 +113,11 @@ class MimeType final {
   static constexpr bool isXml(const T& mimeType) {
     auto type = mimeType.type();
     auto subtype = mimeType.subtype();
-    return (type == "text" || type == "application") &&
-        (subtype == "xml" || subtype.endsWith("+xml"));
+    // Bare "xml" subtype is only valid for text/xml and application/xml.
+    // The "+xml" structured syntax suffix (RFC 6838 §4.2.8, RFC 6839) indicates
+    // XML-based content regardless of top-level type (e.g. image/svg+xml).
+    return ((type == "text" || type == "application") && subtype == "xml") ||
+        subtype.endsWith("+xml");
   }
 
   template <IsMimeType T>