(core) Fix reporting of blank attachment types

Summary:
The conditions in the map/filter of attachments was faulty, causing
blank attachment types to slip through.

Test Plan: Tested manually.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3865

app/server/lib/ActiveDoc.ts

@@ -2326,8 +2326,9 @@ export class ActiveDoc extends EventEmitter {
     const attachments = this.docData?.getMetaTable('_grist_Attachments');
     const numAttachments = attachments?.numRecords() ?? 0;
     const attachmentTypes = attachments?.getRecords()
-      .map(r => r.fileExt?.slice(1) ?? null)
-      .filter(ext => ext !== null);
+      // Exclude the leading ".", if any.
+      .map(r => r.fileExt?.trim()?.slice(1))
+      .filter(ext => Boolean(ext));
 
     return {
       numAttachments,