Document required TypeScript libs

Version 13.0.0

I get in the new version the error:

```
<sourcefolder>/node_modules/apollo-upload-client/node_modules/extract-files/extractFiles.mjs(141,30)
      TS2488: Type 'any[] | FileList' must have a '[Symbol.iterator]()' method that returns an iterator.
```

It seems like a bug in your library in combination with typescript.

I use the library via apollo-upload-client