every post back of client script fileupload data remove?

hi any help, how can i fix my error I have a text box and a fileupload, every time i choose a file and then save i forgot to add a text in text box i had a error meesage box but when it prompt then i click ok the data that i choose in fileupload was remove. how can i fix this that i wont remove? thanks im using asp.net c#

heres my client script
 ScriptManager.RegisterStartupScript(this, this.GetType(), "script", "confirm('Please Enter Description!');", true);
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