Top menu >> Processes >> Drafts
You have the option in the setup of a process template to make attaching a file in a folder required. This means that when you start a form and do not attach a file and put it in the required folder, the form will be added to the Drafts section.
When you start a form and cannot attach the file you have the option to either Discard or Next. Discard removes the form, when you click Next the form will be placed in the Drafts section.
When you + Add the file or link make sure you add it to the folder that has the red File required warning, in the example below 'Signed Contract'.
Click Finish when done.