To enable the ability to add attachments to a model driven form you need to make sure the entity supports notes/attachements. This will start showing the timeline control where you can add a note and attach the spreadsheet or other attachment you mention.
In the Entity editor got to settings for that entity check the "Enable attachments (including notes and files)" checkbox.
Once this has been checked you need to add the "Timeline" control to the form. Open the form in the classic designer. Select the "Insert" tab on the ribbon and find the "Timeline" button. It is at the bottom of the stack below "Web Resource". Move this control where you want it then save and publish your form.
You should now see the timeline control on your form. Clicking the paperclip icon will load the attachment dialog.
Attached image of the timeline control below.
Hope this helps,
Rich
