What's this button for?
(Also, a "help" page might be really useful here ...) (Or somewhere ...)
I believe you are seeing this button when you start to compose a message on the Community. On my computer it is showing as "Choose File" but this could be due to varying OS's or browsers. Clicking this button allows you to add attachments to your posts in the Community.
You can also access the help page by clicking on your avatar/profile image towards the top right of the page and selecting "Help." I have also linked the help page here to make it easier.
If you have any further questions, let me know!