Nintex Automation Cloud - Add attachment to an item on-prem

Yes there is a way in online , i don’t now if it will work in On-premise, 

1. Create a long texte field on your SharePoint list (Html)
2. Create a library in SharePoint where you will store your file 
3. In Nintex automation cloud , use a for each action in your Nintex form to get your file collection , choose file as data type  ,
4. In Nintex automation ,with the sharepoint  action Store a file, store your file in the library created in step 2, create a variable for the result Output (Ex : varMyFiles)
5. In Nintex automation , with Create a text string action put this code in the string Components: 

 varMyFilesUrl
<a href="{"valuePath":"$['Url']","key":"c_varMyFiles"}" target="_blank">{"valuePath":"$['FileName']","key":"c_varMyFiles"}</a><hr>

Store your string components result in a variable name it varMyFilesUrl

6. In Nintex automation cloud if you have used create an item in sharePoint list action , Go and search for your sharePoint field created in step 1 , and give the value as varMyFilesUrl

If you want to display the attachement in your validation form use a label action in the form and put your varMyFilesUrl inside 

I wish that can help u 

Happy New year To you