Add comments to: AttachmentViewMain.swift

It would be nice for everyone participating in this year's software project or those to come to have comments explaining the files a little bit.

I will make the start with 'AttachmentViewMain.swift'

I hope that you will create similar issues where you just add comments to files you worked your head around.

Here you find a Swift Style Guide by Google:
https://google.github.io/swift/#documentation-comments

I am looking forward to read more comments by you ;-)