There is a long lag while the attachment is being sent as indicated by the progress bar at the bottom of Mail's "Mailbox" column and during that time the original file that was attached to the message is locked. Eventually you hear the sent mail swoosh (assuming you have it tuned on) the message is transferred to the [I]Sent{/I] mailbox and the original file is unlocked.
"All you've got to do is own up to your ignorance honestly, and you'll find people who are eager to fill your head with information" --Walt Disney
