Typescript can't import files without extension

TypeScript (at the time of writing in the default classic resolution setting) will automatically detect files that end in .ts or .tsx and not require an extension in those cases. All other files require an extension as per TypeScript module resolution conventions: https://www.typescriptlang.org/docs/handbook/module-resolution.html

Tl;DR: If you include A without an extension TypeScript will look for A.ts or A.tsx.