Solved.
add_filter('wp_handle_upload_prefilter', 'custom_upload_filter' );
needed to be assigned a priority of 11 or higher (since the WP default is 10).
So the following code now allows me to rename the filename of a media file uploaded through my some_file_upload
field, which is an ACF File Upload field:
// ACF Upload Prefilter:
add_filter('acf/upload_prefilter/name=some_file_upload', 'acf_upload_prefilter' );
function acf_upload_prefilter() {
// WP Handle Upload Prefilter:
add_filter('wp_handle_upload_prefilter', 'rename_current_acf_upload', 11, 1);
}
function rename_current_acf_upload( $file ){
$file['name'] = 'custom-filename-' . $file['name'];
return $file;
}