Hello Scott Louque,
Enabling attachments is something that you can do on each layer within a Hosted Feature Service.
If you go to your Content in ArcGIS Online and select your service of interest, within the "Overview" section, you should see a "Layers" section. Here, you should see an option to "Enable Attachments".
Attached to this response is a screenshot illustrating what I'm talking about.
Let me know if you have additional questions!
Joe