How to resume interrupted download in i.files app
No one can really answer this question except the developer, it will just be opinion based. So here is my opinion:

This is a developer issue, you can review the app and tell them about this bug. This app doesn't have a support page in English so I doubt you will get support from them directly.

Here is an alternative app that can resume downloads:

Downloads Free | Downloads

Please note that this app can not save video files any more, that's why the current version has 1 star rating. But other than that, it's perfect! :)