FEATURE: Make asset validation compatible to Neos UI / backend modules

[nezaniel]

Is there an existing issue for this topic?

• I have searched the existing issues

Description

We have a requirement where assets need custom, backend-side validation, i.e. file size or image dimensions etc.
I've tried two approaches so far:

1. AOPing into the respecting controller actions to validate the file and add a flash message
   Effect: Validation works, but the flash message gets lost $somehow
2. Adding a custom implementation of the AssetValidatorInterface
   Effect: Validation works, but Fluid cannot handle the FlowUploadedResource that then seems to be the asset's resource property then.

Possible Solution

It would be nice to have additional control over the upload mechanism from project packages, i.e. by configuration or implementing an interface etc.
Probably of particular interest for the new Media UI