Please read the documentation first.
After a user uploads a file through your contact form, Contact Form 7 checks to see if: 1.) Any PHP errors have occurred; 2.) the file type and file size are valid; and then, if the check turns out okay, Contact Form 7 moves the uploaded file to a temporary folder. At this point, Contact Form 7 attaches the file to the mail and sends it. After these procedures, Contact Form 7 then removes the file from the temporary folder.
You would make a big security/privacy risk if you left files uploaded through a public contact form.