Function getLanguageFromExtension

src/constants.ts:389–399

(
  ext: string,
  override: Map<string, string> = EXTENSION_LANGUAGE_MAP,
)

 * matches what a native file of that language would get.
 */
export function getLanguageFromExtension(
  ext: string,
  override: Map<string, string> = EXTENSION_LANGUAGE_MAP,
): string {
  // Normalize so a caller passing an uppercase ext still matches the
  // (lowercased) override keys; the only case-sensitive built-in key, `.R`,
  // collapses to `.r` with the same value, so this changes nothing else.
  const normalized = ext.toLowerCase();
  const target = override.get(normalized) ?? normalized;
  return EXTENSION_TO_LANGUAGE[target] || "plaintext";
}