we want to make sure that Sales People upload the signed Quote Document and ideally make this trackable.

We would expect that there is a lookup type field for Documents but it seems there isnt. 

Something that basically every tool has where users can directly Browse their documents. 

Copy & Pasting the SharePoint Link to the document in a text field would be a real shitty and low usability workaround.

We are using the Sharepoint integration.

Any smart idea how we can check if they uploaded the signed quote?

Maybe some HTML webresource that we embed and a JS saves the file to Sharepoint?