Attachment previews are disabled for certain file types in the legacy editor

Issue Summary

Known affected file types:

• PDF files
• Excel (.xlsx) files

Previewing is disabled for attachments in the following places:

• Attachments inserted into legacy page content as a thumbnail or link
• Attachments macro
• Attachments menu (... -> Attachments -> Files)

Steps to Reproduce

1. Create a page in the legacy editor.
2. Insert a PDF attachment.
3. Publish the page.
4. Click on the attachment thumbnail to open a preview.

Expected Results

A pop-up should display a preview of the PDF file.

Actual Results

A warning flag is displayed:

The preview feature is temporarily unavailable.
We're working on it. In the meantime, to view the file you can download it by going to ... > Attachments > Files.

or the "Preview" button is unavailable.

Workaround

• Use the PDF macro to display an inline preview.
• Download the attachment in the `(...) -> Attachments -> Files` menu.
• Convert the page to the new editor.