You shouldn’t doubt about security and reliability as all your messaging and file exchange transactions run on your own server. You can find all the instructions about server installation here: https://github.com/metarhia/server/blob/master/README.md
You can exchange any files without limits of size. Once you upload the file, you receive its code. You can download the file by code from server and it gets deleted from server right after being downloaded.
Chatting is very secure as well: to chat with someone, you create a room with specific name. One does not simply enter your chat room and see your conversation as there can only be two interlocutors in the room. All the files you transfer in chat are sent right to your interlocutor and are not ever stored on server. Once you leave the chat room, all messaging history is deleted.
MetaCom is absolutely free and contains no annoying ads. It’s minimalistic and easy to use.