how can I show a default value in a Field Properties (File) element?

A default value is not available for this for component because the selection is from the users device, not from within Appian. What is your use case? Maybe we can think of another method to accomplish it.

A user uploads a file using this field.
In the next form a user should see this file, and if he wants to update it he should be able to.
The file is a required object, so if he doesn't want to update, he doesn't have to do anything.

On form1- Use a file field component to capture a new file. Store in appropriate place in Appian. "Save Value As" ac!document. Save the form, and map the ac!document to pv!document on the data tab.
On form2: Use a link component and populate with linktodocumentinternal(pv!document). Follow with an optional file form component, select "New Version". Document to replace=pv!document.