"Add attachment/file" button for a mobile app?

I'm re-designing a part of a mobile-first app for reading and answering customer feedback.

My problem is: How would you display a button for uploading attachments/files in an interface that is going to be mobile-first? Adding a file to the post is optional for the user. Sorry for the rough sketch of the view, but the idea is that there is a comment box for text input on the top, and three different action buttons below (this is not my design, and for now I can't really change anything other than the attachment-button situation).

Adding an attachment isn't a primary action like the other three actions, so what would be the smartest way to approach this? Where would be the best spot to place it in so that the user can't tap on it by accident? Are there any standards for file uploading on mobile?