This is likely more easily done in some other programming language. For instance, you could have a bash script running in the background that uses rsync to copy data from Bela to the server.
Where is your server? If it's on the same network as Bela, the script could even run on the server itself. If you do not have console access to the server and you want to, e.g.: do a POST request, then this could be done easily with curl from the command line, or using libcurl to do it from C++. I never used libcurl, but my guess is that the command-line option would be easier. If you don't want to set up a script to watch for file changes, you could even use system() to run your script from within the C++ program (e.g.: from cleanup()).
