I was able to find the solution to this problem.
Assuming you’re able to SSH into your server, do the following:
1. cd [Magento install folder]/downloader/lib/Mage/Connect/Command
which in your case would be
2. As a stop-gap, check to make sure the problem files do exist by running this command:
You should see a list of files like this:
._Channels_Header.php ._Channels.php ._Config_Header.php ._Config.php ._Install_Header.php ._Install.php ._Package_Header.php ._Package.php ._Registry_Header.php ._Registry.php ._Remote_Header.php
These files are duplicates which have the strange characters in them. If you run “grep ‘com.apple.quarantine’ ._*” (which searches the file contents for the quoted string), you will probably see these files listed.
3. Delete these bad files:
4. Logout as you’re done. You should be able to login to Magento Connect.