COPY FROM - how to identify results?

I've written a web application where users can upload spreadsheets,
instead of having to key in forms. The spreadsheets get parsed and
INSERTED into a table, and with the INSERT gets added an identifier so
that I can always trace back what a particular row in the table
corresponds to.
I'd like to use COPY - FROM to achieve the same thing, but a stopping
point is that I don't see how to add the new spreadsheet with a
particular identifier.

I'd like to be able to do something like
COPY mytable (field-1, .. field-n, id = my_id) FROM file; or
COPY mytable FROM file WITH id = my_id;

A very messy solution would be to create a temp table with a special
name, COPY to it, then INSERT from it to the permanent table. However, I
don't want a solution of that type.

I assume many people have this same problem. Any elegant solutions here?

Thanks
Jaime

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