I'm wondering if anyone has had a case in which attachment in SharePoint form shall be attached in email notification.
Your suggestion is always very much appreciated.
By SharePoint form, do you just mean a List item with an attachment?
If so there are a number of methods available through webservices for working with attachments, there's some information in one of my blog posts that might help.
With regards to attaching the document (also consider whether a link to the document might be better) are you using O365 or on prem?
Looking at the o365 options below, it looks like you can provide a link to the document you wish to attach?
If you map your site as a network drive, then when you explore your library, and "show hidden folders" you'll see an attachments folder, this can give you the link to your attachment.
I haven't tried this myself, but in my head, the theory is there.