Usability problem: update_file accepts very large files, and binary files 

I made an error and instead of `put_file` used `update_file` for sending large 3GB gzipped file to server.

Whole Provy used about 12GB of RAM, but sent the file properly. 

I guess we could issue an warning (error?) if someone uses this method to upload large binary file, as it does not make sense. 

What do you think about it. 
