Create Custom Validaton Rule

If the built-in validation rule does not cover your requirements, you can create your custom validation rule. In order to do this you need to create a class deriving from the native ValidationRule and override its Validate() method. In that method you can implement custom logic validating the selected file. Since, you might have troubles with some cloud providers while uploading files containing underscores in their name, it might be reasonable to validate their name like this:

public class MyValidationRule:ValidationRule 
{ 
  public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo) 
  { 
      var fileName = (string)value; 
      if (fileName.Contains("")) 
      { 
          return new ValidationResult(false, "You can not upload files containing underscore ('') in their name."); 
      } 
      return new ValidationResult(true, null); 
  } 
} 

Public Class MyValidationRule 
    Inherits ValidationRule 
    Public Overrides Function Validate(value As Object, cultureInfo As System.Globalization.CultureInfo) As ValidationResult 
        Dim fileName = DirectCast(value, String) 
        If fileName.Contains("") Then 
            Return New ValidationResult(False, "You can not upload files containing underscore ('') in their name.") 
        End If 
        Return New ValidationResult(True, Nothing) 
    End Function 
End Class 

Once this class is creted, you need to include it into the ValidationRules of the RadCloudUpload control. This can be done in XAML like this:

<telerik:RadCloudUpload> 
  <telerik:RadCloudUpload.ValidationRules> 
      <local:MyValidationRule /> 
  </telerik:RadCloudUpload.ValidationRules> 
  ... 
</telerik:RadCloudUpload>