Changed files graphql query to use a filter argument to look more like other queries.

graphql/schema/types/file.graphql

@@ -29,6 +29,15 @@ type File {
   url: String
 }
 
+# Filter for the files quiery.
+input FilesFilter {
+  # If set, this will limit the results to either public or private files.
+  public: Boolean
+
+  # Limit the MIME types of the files.
+  mimeTypes: [String!]
+}
+
 # Input for editFile mutation
 input EditFileInput {
  # The file's unique ID