Validate map of known and unknown key-values with Spec

I want to create a clojure spec to validate :multipart parameter of an http request, created by reitit.ring.middleware.multipart middleware.

The multipart form data must contain particular parameters, which could be validated with s/keys, and any number of files with arbitrary parameter name.

The map to validate would look like this:

           {:visualisation "vis"
            :file-xy       {:filename     "foo.png",
                            :content-type "image/png",
                            :tempfile     "C:\\Temp\\ring-multipart-123.tmp",
                            :size         295281}
            :file-abc      {:filename     "bar.png",
                            :content-type "image/png",
                            :tempfile     "C:\\Temp\\ring-multipart-456.tmp",
                            :size         42}}

I can validate the files with reitit.ring.middleware.multipart/temp-file-part spec like this:

    (s/def :multipart/files (s/map-of :multipart/param multipart/temp-file-part))

Putting it together, I came up with a spec which passes, but it allows all unknown parameters to be either file or string:

(s/def :multipart/param keyword?)
(s/def :multipart/visualisation string?)
(s/def :multipart/items (s/map-of :multipart/param (s/or :file multipart/temp-file-part :visualisation string?)))
(s/def :visualisation/files (s/and (s/keys :req-un [:multipart/visualisation])
                                   :multipart/items))

How can I define a spec for a map with specific keys and a value validator for other keys?

You could add a predicate function to ensure only one of the map's values is a string:

(s/def :visualisation/files
  (s/and (s/keys :req-un [:multipart/visualisation])
         #(= 1 (count (filter string? (vals %))))
         :multipart/items))

(s/conform :visualisation/files
  {:visualisation "vis"
   :file-xy       {:filename "" :content-type "" :tempfile "" :size 0}})

(s/conform :visualisation/files
  {:visualisation "vis"
   :file-abc      "not valid"
   :file-xy       {:filename "" :content-type "" :tempfile "" :size 0}})

But the catch-all s/or spec will produce redundant tags if you're using the conform'd output.

Another option could be to use two spec checks:

(def data
  {:visualisation "vis"
   :file-xy       {:filename "" :content-type "" :tempfile "" :size 0}})

(s/conform (s/keys :req-un [:multipart/visualisation])
           data)

(s/conform (s/map-of :multipart/param :multipart/temp-file-part)
           (dissoc data :visualisation))

If you control the shape of the data or can rearrange it, I'd consider something like this which would be easier to spec:

{:visualisation "foo"
 :files {:file-1 {,,,}
         :file-2 {,,,}}}