Non admin users cannot upload file attachments to a child object of Project, but only in the Modern UX
They get the error: "API-1007: You are not authorized to process this request"
Steps to Reproduce
1- In Studio, create a new object called: "Client" that has "Project" as its Master Object
2- API enabled = Yes
3- Add the following Attributes. The important one is the Attachment Data Type

4- Save and Return
5- Continue to the Views Tab, select "General" > "Layout Edit"
Include the "Client Attachment" attribute
7- Open a Project in the classic UI and go to the Properties tab drop down and select "Client List"
8- Create a new Client and notice that it will allow you to upload a file attachment
This does not work in the Modern UX
9- Login to the Modern UX
10- Administration > Blueprints
11- Edit or copy an existing blueprint
12- Under the Modules tab, click Edit
13- Bring over the "Clients" module
14- Publish
15- On the Blueprint List form, http://serverName/pm/#/admin/blueprints
drag over an existing Template to the Blueprint you just created
16- Go to "Projects"
17- Create a new Project From Template (selecting the one you modified)
18- Go to the "Clients" tab and click the plus sign to create a new client
The same user can upload attachments to "Clients" in Classic, but not in the Modern UX