VSCode Extension API - Identify file or folder click in explorer context menu

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.

Regarding obtaining a comprehensive list of context keys: in recent VSCode versions, there's a Developer: Inspect Context Keys command. After executing the command, it lets you pick a UI element:

After that, the dev console opens, and you can expand the logged object that contains a full list of context keys and their current values in this "scope":