Do you want this to be triggered inside the game or in the editor?
I have FString property which specifies some file in game content folder.
For now in blueprint, i must write path by hand. Is there some functionality, to add simple file picker for this property in blueprint editor?
In editor.
Not that i know of, if you seen one in editor in property editor then try to find it’s existance in engine source code, or just tell here and i (or me ;p) will find out how it was done
Otherwise you would need to make your own type (can be simple structure) and property UI for it (same as for example you use UClass* it will show you all classes picker combo box). i never did so you need to reserch how to do it, i can give you some starter hints:
Search for “RegisterCustomPropertyTypeLayout” and “RegisterCustomClassLayout” in UE4 giuthub, it leads to functions that registers cutom property layout in module class (when start up module), it lead to custom property layout class, there seems ot be many examples in UE4 source code, try to figure it out and replicate it
Because Slate don’t use reflection system lot of editor components (like editor for specific asset type) need registered manually on module start up and unreagister on module stop, that general hint if you propbe editor stuff, usally this happens when you see some class without reflection (without UCLASS()) if there such a class and it used soemwhere seamlessly most likely there function that register this usally in module startup, other wise editor would not know how to use it and not even know it exists…
As for open dialog, theres that:
and you access IDesktopPlatfrom by doing this:
IDesktopPlatform* DesktopPlatform = FDesktopPlatformModule::Get();
Thanks for your answer, for now it look way more easyer to write path by hand. 
Any chance there has been a way to do this that was developed in the last 7years?
Here’s something similar I wrote about it!