Add temporary warning to blob.c++ to see if MimeType can be used

src/workerd/api/blob.c++

@@ -5,6 +5,7 @@
 #include "blob.h"
 #include "streams.h"
 #include "util.h"
+#include <workerd/util/mimetype.h>
 
 namespace workerd::api {
 
@@ -59,6 +60,18 @@ static kj::Array<byte> concat(jsg::Optional<Blob::Bits> maybeBits) {
 }
 
 static kj::String normalizeType(kj::String type) {
+  // TODO(soon): Add temporary logging if the type is not a valid mime type.
+  // This does not properly parse mime types. We have the new workerd::MimeType impl
+  // but that handles mime types a bit more strictly than this. We can/should switch
+  // over to it but there's a non-zero risk of breaking running code. We might need
+  // a compat flag to switch. To know for sure, we temporarily add logging here to
+  // see if anyone in the wild is relying on the incorrect parsing.
+  // If we see this log even once in production then we cannot switch normalizeType
+  // for MimeType::tryParse without a compatibility flag.
+  if (MimeType::tryParse(type) == nullptr) {
+    LOG_WARNING_ONCE("Blob created with invalid/unparseable content type");
+  }
+
   // https://www.w3.org/TR/FileAPI/#constructorBlob step 3 inexplicably insists that if the
   // type contains non-printable-ASCII characters we should discard it, and otherwise we should
   // lower-case it.