We confirm this is a bug. FileFieldDefinitionElement
doesn't initialize it's default values properly. The issue is logged and will probably appear as a fix in SP. As a workaround you could actually add such field directly in the configuration files in App-Data.
But how exactly do you plan to use this field? Adding such field to the view of some content item doesn't automatically give the functionality for uploading and associating files to it. We currently have such fields in the Libraries but there is also custom server logic that handles what's going on when the file is uploaded and where it is kept and how it is associated with the content item.
the Telerik team
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