Sometimes you may want to download content to your game from a web server. This is useful for IAPs, daily content, custom user levels etc. However, you cannot save these in the working_directory on iOS or your game will be rejected during the store submission process.
The working_directory in GameMaker games points to <Application_Home>/Documents by default, which is only to be used for documents that the user has created or may want to back up to iCloud such as save files and screenshots. Bonus downloaded content must be saved in a different directory such as <Application_Home>/Library/Caches for the game to pass store submission.
You can save to this location by setting the save location to working_directory+'/../Library/Caches/' for example:
save_location = working_directory+'/../Library/Caches/myDownload.file';
Please take note of this information for developing future games.