See https://community.metabrainz.org/t/unable-to-tell-picard-to-open-files-from-applescript/235520?u=freso :

With OSX 10.12.3 and Picard v1.3.2, it was possible to open a folder via applescript. eg:

tell application "MusicBrainz Picard"
	try
		open pathToFiles
	end try
	activate
end tell

In v1.4, this no longer works. Picard itself still opens, but no files are added. I've also tried this with the dev version mentioned here that fixes the drag & drop problem with 1.4.