silverstripe-framework/tests/forms/TreeDropdownFieldTest.yml

SilverStripe\Assets\Folder:
  subfolder:
    Name: FileTest-subfolder
  folder1:
    Name: FileTest-folder1
  folder2:
    Name: FileTest-folder2
  folder1-subfolder1:
    Name: FileTest-folder1-subfolder1
    ParentID: =>SilverStripe\Assets\Folder.folder1
SilverStripe\Assets\File:
  asdf:
    Filename: assets/FileTest.txt
  subfolderfile1:
    Filename: assets/FileTest-subfolder/TestFile1InSubfolder.txt
    Name: TestFile1InSubfolder
    ParentID: =>SilverStripe\Assets\Folder.subfolder
  subfolderfile2:
    Filename: assets/FileTest-subfolder/TestFile2InSubfolder.txt
    Name: TestFile2InSubfolder
    ParentID: =>SilverStripe\Assets\Folder.subfolder
  file1-folder1:
    Filename: assets/FileTest-folder1/File1.txt
    Name: File1.txt
    ParentID: =>SilverStripe\Assets\Folder.folder1