Allow uploading files during submission of a webform.

Closes #3503564

graphql/webform.base.graphqls

+scalar WebformSubmissionFileUpload
 scalar WebformSubmissionValue
 
 type Mutation {
   submitWebform(
     id: String!
     elements: [WebformSubmissionElement]
+    files: [WebformSubmissionFile]
     sourceEntityId: String
     sourceEntityType: String
   ): WebformSubmissionResult!
@@ -52,3 +54,16 @@ input WebformSubmissionElement {
   """
   value: WebformSubmissionValue!
 }
+
+input WebformSubmissionFile {
+  """
+  The name of the webform element.
+  """
+  element: String!
+
+  """
+  The file to upload. This is usually null and the file is appended via
+  multipart form data maps.
+  """
+  file: WebformSubmissionFileUpload
+}