Good day Tan 

Let us look at two different scenarios:

Designer:

From the designer, if you create or edit your smart object you will see a type you can assign to a data item with the name File. This is a specific data type in K2 designer for the storing of files. When you want a file to be stored create an item named the file in your Smart Object designer and give it a "File" data type (please see the screenshot).

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Kind Regards

Jacques.