After searching keyword "airdrop" on this forum and finding nothing, I'll report that on iOS 13 airdrop consistently fails to work from Mac to iPad when sending .afdesign files. Mac is using OS X 10.12. Airdrop works well in every other respect with other apps. Airdrop alert sounds on iPad however nothing shows up, not even a dialog offering to save to Files. No file can be discovered in the Files app. The Mac reports that the file was sent, so the Mac behaves normally on its end. Airdrop works fine going in the other direction. The same issue happens with Affinity Photo as well. The files that fail to send do have proper suffixes, like ".afdesign." Workaround is to zip the file, airdrop to iPad, unzip there in your app of choice, then locate the file in Files app to open, from within Designer.