Test-SPContentDatabase -Name ABCD_Content -WebApplication
Category : MissingSetupFile
Error : True
UpgradeBlocking : False
Message : File [Features\ExcelServerSite\Microsoft.Office.Excel.WebUI.dwp] is referenced  times in the database [ABCD_Content], but exists only under Windows SharePoint Services 3.0 setup folder. Consider upgrading the feature/solution which contains this file to the latest version.
Remedy: One or more setup files are referenced in the database
[ABCD_Content], but are not installed on the current farm. Please install any feature or solution which contains these files
There was an issue with this Web part and the Test-SPContentDatabase command. You should be able to ignore it and upgrade will work.
It worked for me!