Editor.EditorUtility.OpenFileDialog

string file = EditorUtility.OpenFileDialog( "Select VMF file", "vmf", $"{Sandbox.Project.Current.GetAssetsPath()}/" );
string relPath = Path.GetRelativePath( $"{Sandbox.Project.Current.GetRootPath()}/Assets", file );

if ( !FileSystem.Mounted.FileExists( relPath ) )
{
	EditorUtility.DisplayDialog( "Import failed", $"'{file}' is not accessible!" );
	return;
}