You can use "when": "explorerResourceIsFolder"
.
I had to dig through the code to find it (I was actually writing up a response to say it didn't exist and enumerating the possible clause values when I saw it).
As of v1.10.1:
config.<any_config_path_here>
editorIsOpen
explorerResourceIsFolder
explorerViewletFocus
explorerViewletVisible
filesExplorerFocus
globalMessageVisible
inDebugMode
inQuickOpen
inZenMode
listFocus
openEditorsFocus
resource (Uri information: path, query, scheme, etc)
resourceFilename
resourceLangId
resourceScheme
scmProvider
textCompareEditorVisible
I've submitted an issue to improve the documentation for this.
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