UAssetDefinition

See AssetDefinition.h for more info. Basically a UAssetDefinition represents an top level asset in the editor. You can edit how its viewed and what actions they have (see UAssetDefinition_BlueprintGeneratedClass for a good example).

Make custom class

Mandatory

1. Make a subclass of UAssetDefinition or another subclass (E.g: UAssetDefinition_ClassTypeBase)
2. Override GetAssetClass and return the class you want to support.
3. Override GetAssetDisplayName, GetAssetColor (and OpenAssets if you aren’t sub classing UAssetDefinitionDefault)

Then you can override other functions depending on your needs, like GetThumbnailBrush or GetIconBrush.

Custom icon in Content Browser ? → Override GetThumbnailBrush and GetIconBrush (latter can just call GetThumbnailBrush, same params). See this for an example of storing the slate brush with texture.