You can store files directly in MongoDB documents, however there is a limit of 16MB and the Meteor DDP protocol might not be up to the task (reference..)
There are two atmosphere packages that attempt to fill the gap:
Both are extensively documentend, with the latter one being the lighter one.
They either use the MongoDB GridFS or store files externally on S3 or the server’s file system. They both tie in with the MongoDB style of organising data, extending functionality of collections, by having special ‘file’ collections.
These ‘file’ documents can then be referenced from other documents. With these packages come API’s and systems for security, manipulation, filtering, UI helpers etc.
My eyes drooped and my mind wandered every time I tried to read through the documentation for these packages. It seemed a bit overdone, at least for my purposes.