Problem
While uploading an attachment to an item in a SharePoint list using the K2 NewForm.aspx form and you try to open the attachment before submitting the form, the following error appears:
404 - File or directory not found. The resource you are looking for might have been removed, had its name changed, or is temporarily unavailable.
The file URL is shown as e.g. https://.onk2.com/Runtime/Runtime/1234.docx
Solution
According to Labs, this behavior is currently by design. The reason being that the attachment has not even been uploaded to SharePoint, thus no link technically exists to the attachment. This mimics SharePoint behavior. When using the SharePoint form, a hyperlink is not even presented when uploading an attachment to a list item.
A feature request has been logged to allow users to open attachments that have not been uploaded to SharePoint yet. This is planned to be included in K2 Cloud Update 19 releasing in 2023.