How do I get Thunderbird to show the attachments expanded

Solution 1:

As per this writeup on the Attachment Pane Redesign on MozillaZine, you can add the following to userChrome.css to remove the attachment toggles:

#attachmentView > [collapsed="true"] {
  visibility: visible !important;
}

#attachmentToggle {
  display: none !important;
}

Edit: Corrected filename to userChrome.css

Solution 2:

Right-click on the attachment pane, and click "Initially Show Attachment Pane."

(The option was added in Thunderbird v24 and up.)

Solution 3:

This ability to change this through the UI has been removed in newer versions, but it still exists in about:config (Preferences > Advanced > General > Config Editor...)

Set mailnews.attachments.display.start_expanded to true