To do this, you need to use if/then logic to filter for the files you want. Something along the lines of this (which I did not test, so the syntax may be slightly off)...
on adding folder items to my_folder after receiving the_files
repeat with i from 1 to number of items in the_files
set theCurrentFile to item i of the_files
if (name extension of (info for theCurrentFile)) = "mp3" then
tell application "iTunes"
launch
try
set this_file to (item i of the_files)
add this_file
end try
end tell
end if
end repeat
end adding folder items to
Or, why not use Automator to do this? It has an action for filtering files and an action for importing files into iTunes. And, you can create a Folder Action with it. See my attached screenshot.
Hope this helps.
-Ben