Hi @BrittanyR ,
Regarding the screenshot that you provided, I think you are in right direction. Please go to the Entity form, then switch to "Additional Settings", then scroll down the page, then you could find the "Add Attach File" section there:

Please configure your Entity form as above screenshot.
In addition, please also make sure you have enabled "Enable Entity Permission" option for your Entity form as below:

The "Attach a file" button would shown up as below when you edit an existing record:

Also please check and see if the thread solution I provided below could help in your scenario:
https://powerusers.microsoft.com/t5/Common-Data-Service-for-Apps/How-do-I-enable-a-File-upload-field-in-a-Form-embedded-on-a/m-p/578708
Please try above solution, then check if the issue is solved.
Regards,