Rename 'inApp' field of 'File' and add description

[lgrahl]

The File.inApp field is utterly confusingly named which is probably why it also lacks a description. 😅

Description: It controls whether a file message will be rendered according to the mime type within the app (if feasible). If false, the file should be treated as an attachment that will not be rendered.

It is currently unused in both app and web client. I'd propose to rename this to render.