This is a desktop version feature. When the user is editing a local mind map file, there are two ways to add a attachment file to a selected topic:
1, Drag a local file and drop it on the selected topic.
2, Add a attachment file fllowing these steps:
2.1 Click Edit->Topic feature->Attachments menu item.
in the popup menus, click "select files" to add a local file to temp folder first.
2.2 Click Edit->Topic feature->Attachments->Add Attachment menu item, a dialog will popups up. In the temp file list, choose a file, then click "OK" to add this file to the selected topic.