Is it possible to make a folder look and behave like a file? I'm developing a desktop application which saves its projects to disk.  Projects contain multiple files, like json and jpg etc.

Current solution: zipped file

I'm "zipping" them all together into a single file.  Example:   

filename.project            &lt;- compressed zip
  manifest.json             &lt;- compressed contents
  file1.jpg                 


Ideal solution: uncompressed folder

I'm wondering if it's possible to output the data, uncompressed, to a folder instead, and somehow configure my OS to treat that folder as a single file?  Example:

filename.project            &lt;- folder, but it gets treated like a file
  manifest.json             &lt;- uncompressed contents
  file1.jpg


Definition of "Treated as a single file":


Finder shows a custom icon
Double-clicking opens the file association
"Open" dialogs can select this file


Other examples

On my Mac, there are 2 examples where I see this happening:
 - All apps appear as .app files, but internally are folders that I can explore via Terminal
 - 1Password stores its password store in a .agilekeychain folder, which syncs to Dropbox as a folder, but appears in Finder as a single file.
Apple Developer info says yes, you can definitely make your own package or bundle. Seems as if you want a package.

Package vs. bundle is discussed here.

Basically - register your package with the system, so that folders with the extension are displayed as a package rather than as a folder (application info.plist must appropriately set key LSTypeIsPackage for each of the package filetypes you need to create). Then set your app to automatically (using NSFileWrapper) or manually create the package itself on save.