Fix cross-realm linksTo MIME type handling for file/image defs#4400

lucaslyl wants to merge 2 commits intomaincardstack/boxel:mainfrom

CS-10730/card-thumbnail-linksto-imagedeffiledef-fails-to-save-for-cross-realmcardstack/boxel:CS-10730/card-thumbnail-linksto-imagedeffiledef-fails-to-save-for-cross-realmCopy head branch name to clipboard