How I must give a higher permission to flatpaks? for example I can't send files with Telegram flatpak.
You can do this via the command line. The specific command you need is (it may need to be run as sudo):
flatpak override package_name_here --filesystem=path_here
If you have an odd path & it complains about an "unexpected filesystem suffix", just put the path part in quotes.
A more complete list of permissions commands is found here - http://docs.flatpak.org/en/latest/sandbox-permissions.html