While your file is stored within a contract on sia, is the file still available for download and usage/editing by yourself?  In the same way I might email myself a document so I can continue working on them from home, or some people use Dropbox for this same purpose. 

I'm not sure how this could be possible (everytime you edit the file it would surely change the file and its size, thus making your contract invalid).  If it isn't possible then that would make sia more like a bank savings account for your files, where you can't touch them for a particular period of time but they are indeed kept safe. 

I obviously know nothing so any explanation is appreciated

Cheers,
Calum