Title says it all.
Also, you can’t open an attachment if the cursor is already “inside” the item containing the attachment. You must click away and then click the link to get a response.
For attachments that point to a file in the local OS file system, the “open” response can be rather slow, so at first I actually thought that Legend couldn’t open those attachments at all. I had tried regular-clicking the file but it took so long to open I already had the title for this post written before I saw that the file had actually opened. This was with a 1kb text file so response should have been much quicker.