One Space
Replace XYZ with your space and tempdir with the directory you want to put the attachments in.
--action runFromAttachmentList --space XYZ --common "--action getAttachment --id @pageId@ --name \"@attachment@\" --file tempdir" --continue
All Spaces
Replace tempdir with the directory you want to put the attachments in. This can take a long time on larger sites!!!
--action runFromAttachmentList --space @all --common "--action getAttachment --id @pageId@ --name \"@attachment@\" --file tempdir" --continue
To Avoid Duplicate Attachment Names
Attachment names are only unique for the page they are on, so you might want something more precise.
--action runFromAttachmentList --space @all --common "--action getAttachment --id @pageId@ --name \"@attachment@\" --file \"tempdir/@space@/@title@/@attachment@\" --continue