Right now the only way I can send calendar ICS files or even contact files are by switching the factory texting system back to default. There is no option to send these files through Textra at this time. I really like Textra but it's a pain to have to keep switching back and forth from the factory app to Textra to send these files.
Android messages has this really handy feature that when a security code gets sent to your phone, one of the buttons in the notification (besides reply) turns to "copy #####" where ##### is the code sent to you. you can tap it and it copies the code so you can then paste it where needed