Permissions error when downloading or quick-looking attachments in mail.app after symlinking Mail Downloads directory

Awhile back, I symlinked my ~/Library/Mail Downloads directory to ~/Downloads so that my email attachments would be downloaded to the same location that all my other applications download files to.

When I upgraded to Mountain Lion, I noticed that the Mail Downloads folder had been restored. So I deleted it and recreated the symlink the same way I did for previous versions of OS X.

Now, every time I try to download or quick-look an attachment in mail.app, I see the following error (might not be exact; translating from Spanish):

Mail was unable to save the attachment "…" to disk. Make sure that the downloads folder has write permissions.

Some of the fixes I have tried:

• Making ~/Downloads world-writable.
• Removing the symlink and re-creating the directory.
  • Making that directory world-writable.
• Repairing permissions via Disk Utility.
• Changing the location of the Downloads Folder preference for Mail.app to ~/Downloads.

How do I restore the functionality of my Mail Downloads folder (preferably so that it is symlinked to my home Downloads folder)?