Define a task property as one that can change per platform

I am developing a VSCode extension that provide tasks, how can I specify that a property can change per platform, for example on "shell" task we can have this tasks.json:

"tasks": [
{
    "type": "shell",
    "windows": { "command": "wndCmd.exe" },
    "linux": { "command": "lnxCmd" },
    "osx": { "command": "osxCmd" }
}]

but for mine it is not possible.

For the sample on documentation, it is not possible create a task like:

"tasks": [
{
    "type": "rake",
    "task": "some",
    "windows": { "file": "winFile" },
    "linux": { "file": "linuxFile" },
    "osx": { "file": "osxFile" }
}]