How to Add Format to Existing Clipboard Contents in C#

Thank you for your help.
Sorry for asking so many questions.
You are about to add a new format to the contents of the clipboard that is already included in C#.
For example, an image in which a file is copied to the clipboard and data such as strings are added to it.
Below is the data from the clipboard and I'm trying to add a string to it.

using System.Windows.Forms;

IDataObject data=Clipboard.GetDataObject();
if(data==null){
return;
} else {
data.SetData(DataFormats.Text, content);
Clipboard.SetDataObject(data,true);
}

However, it doesn't seem to work well with this code, and rather than the string being copied, it seems that the contents of the previous copy have disappeared.
When you copy the file and look at the format stored on the clipboard, you will see the following:

 FileDrop, FileNameW, FileName, Preferred DropEffect

After that, if you add string data with the above code, the format list is as follows, but if you actually try to paste a string or a file, you cannot paste it.

 FileDrop, FileNameW, FileName, Preferred DropEffect

Could you tell me what the problem is with the above code?
Thank you for your cooperation.