* perf: avoid making an unnecessary copy of the vector
MimeTypesHandler::GetMIMETypeAllowlist() returns a const&, so we can
iterate that directly instead of making a temporary copy of it.
* perf: move the call to ExtensionRegistry::Get() outside of the loop
Also, keep the previous behavior of not calling it at all if there
aren't any whitelisted extensions.
* perf: avoid redundant map lookup
* refactor: const correctness
* refactor: cleanup
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