Simplex will always place uploaded files in datacgi/files/YourFile/tables/files directories, arranged into a series of subdirectory slots. So, unfortunately, it is not possible to upload to a particular directory with the usual set up.
If you really need to upload to a specific directory with specific file names, you would need to edit the Perl code to do so. If you're familiar with Perl, this shouldn't be too difficult, and you can check the datacgi/libs/simpsubs.cgi script from line 940 to 1000, and datacgi/libs/adminsubs.cgi from line 361 to 424 for the relevant code. The first code segment is used when uploading files through a form, and the second when uploading files through a table datasheet from the admin system.