Add file domain utility function to decode obj with schema

[Re #1543]

* Useful if we want to convert a file to a domain when pulled from the DB using join
and JSON aggregations.

backend/src/gpml/domain/file.clj

@@ -3,7 +3,8 @@
             [gpml.domain.miscellaneous :as dom.misc]
             [gpml.domain.types :as dom.types]
             [gpml.util :as util]
-            [malli.core :as m])
+            [malli.core :as m]
+            [malli.transform :as mt])
   (:import [java.io File]))
 
 (def ^:const object-key-pattern
@@ -55,6 +56,10 @@
       (str/replace #"FILE-KEY" (name file-key))
       (str/replace #"FILE-ID" (str file-id))))
 
+(defn decode-file
+  [file]
+  (m/decode file-schema file mt/string-transformer))
+
 (defn base64->file
   [payload entity-key file-key visibility]
   (let [[_ ^String content-type ^String content] (re-find #"^data:(\S+);base64,(.*)$" payload)