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

Show Alert Message in ASP.Net from Server Side using C# and VB.Net

I hope this will help you out.

thank you!!!!!