Can you write "Hereby attached your reservation confirmation..." in an email?
I do not know if it is better to say: Hereby I send your reservation confirmation attached. Or something like that. Thank you for your help!
P.S. The reservation stuff is just an example. I am more concerned with how to express the "attached" part.

Question

Answer

I would use:
"I am attaching the confirmation for your reservation."
or
"I have attached the confirmation of your reservation"
or 
"Attached please find your reservation confirmation"  (if it's a very formal, very business context.)

I would not use "hereby."