Editor 'Insert Image' dialog returns an error despite image being successfully uploaded

Three different methods can be used to upload attachments to Confluence:

1. Drag and drop (calls /plugins/drag-and-drop/upload.action)
2. View attachments page (calls /pages/doattachfile.action)
3. 'Insert Image' dialog from the Editor: Editor > Insert > Image (calls /pages/attachfile.action)

The 'Insert Image' dialog performs the upload via an AJAX request which has a default timeout set to 12 seconds. Large files uploaded via the 'Insert Image' dialog will return an error "Could not upload the file to Confluence. The server may be unavailable" if the time taken to upload the file exceeds 12 seconds. This message is displayed despite the file being uploaded correctly.

The error message does not appear for the other two upload methods.