Add the possibility to append additional validation callbacks in Magento\Config\Model\Config\Backend\File

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### Description (*)

Consider the case of a custom file upload.
We can extend the File class and add, let's say, a specific file extension via _getAllowedExtensions
Why we shouldn't be able to add one or more callbacks for validations too ?
At the moment the only way to do it, it's to completely override the beforeSave() function as we have no method (or property) we can use to set an array of additional callbacks.
The method addValidateCallback is called to add only "file size validation"

### Expected behavior (*)

A way to specify additional validation callbacks

### Benefits

Cleaner code
Less overridden magento code

### Additional information

Start by inspecting Magento\Config\Model\Config\Backend\File, especially beforeSave() and addValidateCallback(). Determine how validation callbacks are currently stored and invoked, then define the registration behavior needed for additional callbacks. Done means custom file-upload validation can be added without completely overriding beforeSave().

Written by the indexing model from the issue text.