AppleScript Error -1728 Can't get file...

I have tried to customize the AppleScript template “Mail Rule Action” for my purposes. I want to extract the attachments from the e-mail that match a rule.

To do this, I have set up a rule in the Mail program which then calls the following script.

using terms from application "Mail"
	on perform mail action with messages these_messages for rule this_rule
		tell application "Mail"
			set the message_count to the count of these_messages
			repeat with i from 1 to the message_count
				set this_message to item i of these_messages
				
				set attachment_folder_path to (path to downloads folder) as string
				
				tell this_message
					try
						repeat with att in this_message's mail attachments
							set org_name to the name of att
							save att in file (attachment_folder_path & org_name)
						end repeat
					on error error_message number errNum
						display dialog "Some error: " & errNum & return & error_message
					end try
				end tell
			end repeat
		end tell
	end perform mail action with messages
end using terms from

Unfortunately, I always get the following error message.

Can’t get file "Macintosh HD:Users:testuser:Downloads:test_file.pdf"

The error number is -1728.

How can I solve the problem?