Hi,
You can simply check Dodopdown Selected Value and if it's value Registered Company",then you can check if file uploader has file
refer below article to check selected dropdownlist
Save (Insert) DropDownList Selected Value to Database in ASP.Net using C# and VB.Net
and check fileupload value like below
C#
if(fileUpload1.HasFile)
{
}
also after checking both condition and you show message using below article
I hope this will help you out.