I am not sure if this is an issue that is isolated to the DropBox provider or to all of them. But the issue is when you have multiple attachments and then try and use FileLink to upload them, it appears like ThunderBird tries to make too many connections at the same time or tries to upload too many of them at the same time because some of them will go through but then all of a sudden an error message pops up for the remaining files that says "Upload Error" Unable to upload FileName to DropBox. Which you could then right click each item below at the attachments location and Convert to DropBox but you have to do it with each individual file. It seems like maybe the FileLink feature itself just needs to space out each of the uploads in order to avoid this error but I am not sure.