How to add a file to a message as an attachment?

Pretty please. I've tried to use something similar to

file.inputStream()

file.inputStream()

but to no avail, and I can't find how to do this anywhere. I have a

File

File

(and its path as a

String

String

) and I wanna attach it in a

createMessage

createMessage

. That's it.

Edit : forgot to mention but I use Kotlin, not Groovy