How to setup default downloaded content emails

Like most things Marketo, there's definitely a number of ways to skin this cat.  Here's one approach I've taken in the past that's worked well --

Build your global content-download autoresponse email.  Use tokens for anything that's going to very from content to content -- what I did was create custom fields for "Downloaded Asset Type", "Downloaded Asset Title", "Downloaded Asset Link", and "Downloaded Asset Thumbnail".

So the email would look something like this:

Hi {{lead.FirstName}}, thanks for downloading our {{lead.DownloadedAssetType}}, "{{lead.DownloadedAssetTitle}}".  You can download it here: <a href="{{lead.DownloadedAssetLink}}"<img src="{{lead.DownloadedAssetThumbnail}}">.

You get the idea.  So within each content program, you have a Smart Campaign update those fields with the relevant information upon form submission, then send the global email you've created.  There's a couple of other ways I can think of that will get you a similar result -- if you don't want to create custom fields, for example, you can use local program tokens and include the email as part of your basic program template.  You wouldn't have a true single email, but you wouldn't need to edit the individual email for every program as long as you were updating the tokens on the Program level.