How to open Text File and then insert line in first position?

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Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Content-Disposition: inline

Hi,

I need to add a header to files, but I didn't find how open files in append
mode ('a') but positioned in first position.
Does anyone know how to do that?

Thanks


-- 
Pablo Q.

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