I would like to have a folder in my media library that can only contain MP3s, or Folders of MP3s. The only way I can think of doing this is by creating a new "MP3 folder" type that includes itself and MP3 files in its insert options. Is there another way that's more elegant? Will what I describe even work?
You can create a new template of
MP3 Folder type and set the insert options on that template's standard values. If you don't want to create another template, you can use the rules engine and create an insert rule where that specific folder or folders are assigned specific insert options.
That being said, insert options do not provide hard enforcement for only those types, that is, a content author can still add other types by using the
Insert from template option. If you want more strict enforcement, you'll need to patch a number of pipelines to ensure that users can't upload non-MP3 files to that item tree or move non-MP3 files into that item tree.