Your calculation will be roughly right for a small number of downloads. But it's not so simple. Assuming a packet size of 1500 bytes (which is the norm for an ethernet network), a 10 MB file is 10485760 bytes. So dividing that by 1500 gives 6990. So transmitting that file ads an additional 6990 packet headers.
Now I'm not sure of the size of a packet header, but they are small, also, if you want to take upload bandwidth into account then you need to allow for the ACK packets of TCP. So I'd just allow a 10 byte overhead per packet as a rough guide, which gives roughly a 68Kb overhead to each 10 MB file.
In addition, if the file is being sent by email, you will need to allow for the encoding so allow another 50%. If being sent via FTP, you need to allow for the overhead of the control connection used to send FTP commands and for the initial TCP handshake. So I'd allow 70k overhead per file downloaded as a rough estimate.