I'm trying to figure out how setup MT as TFTP server to serve few files. Could you give some guidelines were to put files and how to configure? Could I just upload file to "Files" and then somehow configure TFTP to make them available?

you upload files to the router and can place them in some folder or additional storage that is available in router.

then you add /ip tftp rules that allow access to the files in some way.

tftp server will be started only upon tftp rule addition.
http://wiki.mikrotik.com/wiki/Manual:IP/TFTP

you upload files to the router and can place them in some folder or additional storage that is available in router.

Is this possible on RB750G (http://www.routerboard.com/pricelist.php?showProduct=90)? This hardware do not have additional storage option. I see the only way to upload files - put it in "Files" by drag'n'drop or WinSCP, but could not figure out what is their path to configure this in TFTP.