File Sharing is possible. You'd have to upload the file onto the file system of the server to actually share with anyone, but it can be done. Actually, Doogs was hosting his file sharing game yesterday.
BYOND does not support proper file-sharing anyway... the best it can do is act like an ftp/http download service. true filesharing is a distribution/resource-sharing of *pieces* of files from many people so that your own download of said files happens faster. BYOND is just not setup for that, and will likely never have that ability.
*devotes all of his life to giving byond this capability*
Though in theory couldnt a person just have a folder, make BYOND look at the folder when your hosting. Now when someone hits upload it sends the file to folder. When someone click search download you could have a question asking what type of file is it then it would respond and list files. Then you could click one the host would be forced to upload it and then the person downloading, download it? sorry i havent dealt with DM in a long time this may only be applicable on C++ and VB.
sure you *could* do that probably, but why would you? a real ftp service on your computer would do the job much better/faster than trying to get BYOND to do it.
leave BYOND to game-development and let existing free software for file-sharing and file-transfers do the jobs they are designed for.
BYOND does not need to re-invent the wheel on this.