(core) Don't treat Attachments as RefList in creator panel

Summary: Fixing bug introduced in https://phab.getgrist.com/D3338: column config now shows "DATA FROM TABLE" under the Attachments column type as if it were a normal ref/reflist column.

Test Plan: manual

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3350

app/client/widgets/FieldBuilder.ts

@@ -121,7 +121,8 @@ export class FieldBuilder extends Disposable {
 
     // Returns a boolean indicating whether the column is type Reference or ReferenceList.
     this._isRef = this.autoDispose(ko.computed(() => {
-      return isFullReferencingType(this.field.column().type());
+      const type = this.field.column().type();
+      return type !== "Attachments" && isFullReferencingType(type);
     }));
 
     // Gives the table ID to which the reference points.