You can check the following location to view the attachments:
(root)/data/data/com.google.android.apps.inbox/files/downloads
In this location you'll see a random code something like cd8ca9801230132321232sd312312ad312
Navigate to that folder and you would see few other folders named
- attachments
- avatars
- smart_hero_mail_image
- smart_mail_image
To view the attachments, navigate to "attachments" folder. Here you'll find lot of folders with random name. Sort the folders based on date, so you could file the most recent ones on top/bottom. The attachments will be stored on random folders, based on when (date) you opened the files via Inbox application.
Hope this helps.