Thanks so much for letting me know about this. Good eye ??
This is actually from a piece of legacy code when we were showing the category filters in the attachment filters bar.
The next update will have this custom CSS removed, so the search input will have its original width.
I am going to close this issue for now, as it will be fixed in the next release.
If you spot anything else or want to suggest a feature for HappyFiles, then the official idea board over at https://happyfiles.io/roadmap/?tab=ideas is the best place for it.