You now have the possibility to choose for your attachments columns if you want to accept only one document. 

Example

I want to store my invoices in TimeTonic. So I use the single document option to keep only the first invoice generated. You can also combine this method with the read-only line option to lock the information for invoice generation. 

Process

For that, nothing could be simpler. 

Go to modify the attachment column then check "accept only one file" : 

Your column is now configured to accept only one file. It is the first saved file which is authentic. 

An attempt to add a file when a file already exists will display this error message :