Curious as to why you store the data in the database in b64 as opposed to files on disk. What's the reasoning for that? Doesn't it make storage/backups/etc more complicated?
Not an expert myself, I opted for in browser encryption, in chunks, so as to avoid memory limitations (at least in some browsers, not FF yet), and in browser gzip so as to keep file size down and speed things up.
I find your niche quite interesting (journalists, whistleblowers) but given the high stakes of that perhaps an open source or more collaborative approach would be easier to promote.
Another idea I've tried out but not pursued, is some sort of browser extension/addon (I used nwjs, similar to electron), that offers client side encryption for any site (form field really). So you'd only post encrypted stuff to whatever service (email, reddit, hn, whatever) and only anyone with the key would get to read it (well, assuming they have the key and the same extension). Just throwing the idea out there, I'm sure others have thought about something along those lines before. The details to get it right are tricky (UX wise), but for your target audience it may be well worth the extra work.
Keep it up!:)