Hi Anita - welcome!
If this customer/file number field is truly related to customers you'll want to add a field to the customer table in order to hold this data. You can dot his from Customize>>>Tables / Fields and locate the Customer table.
Once that's done, this field will be available on the New Leads screen and the New Customer screen. However, you will also want to add it to the screens that allow you to edit the lead and customer. Since all these screens are built off the same table its a simple drag and drop customization. :)
Since its a customer/file number, chances are its for the main customer (or lead) only so you won't see this available on the Leads and Contacts List or Customer and Contacts List because those grids are built off the contact table. There are ways to list this customer/file number there too..just reply back if that's what you want.
I'll look into the logo question for you next.
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