1. Is it possible to make so that the admin can activate the file-field only for some users? So that if a specific user requests to upload a file, I can allow only that user to upload a file.
Yes, use CB Conditional to condition the file field to be accessible based off whatever conditions you like (e.g. another field or a specific usergroup).
2. How can I make so that I receive a message, notification or an email when a user uploads a file?
File field has no such functionality built in. You'd need to use CB Auto Actions to check for the fields value changing then send an email. See the below topic on how to do this.
www.joomlapolis.com/forum/153-professional-member-support/242902-solved-triggering-an-email-message-based-on-a-field-change-status#320476
3. Can I make so that the file-field only is visible if another field has a value. So if the text-field I choose is empty, the file-field will be invisible. But as soon the text-field gets a text value, the file-field will show up.
Yes, you can do that using CB Conditional.
krileon
