Thank you. You know why I asked the question is because when we file tax we fill out the tax return and mail the forms back to the IRS. So I supposed filing letters is like mailing letters. Obviously I am wrong.
Ah yes, legal and other offical documents are also 'filed', which is sent to the official body. So I suppose that could also be an interpretation here, but it seems unlikely someone would manage to do 100 in a single day?
I think this is the definition which interests here wrt tax, etc:
-------- file
3 a (1) : to deliver (as a legal paper or instrument) after complying with any condition precedent (as the payment of a fee) to the proper officer for keeping on file or among the records of his office (2) : to send (newspaper copy) to a newspaper or news agency by