you wont be able to accomplish this the way you are going about it.
add 6 file form fields to your page, and add six upload file server behaviors to upload the files.
if you insist on using a the File Manager, you will need to have six of them.
in each one, select and image.
Then use the insert record behavior to store the selected file names. each file manager instance can only pass one selected file name, you cannot use 1 file manager instance to pass multiple file names.