Hello Bert,
Thanks for your reply. If I understand, my code should looks like that:
tt_content.uploads.override = COA
tt_content.uploads.override {  
  10 = < lib.stdheader
  20 = USER
  20 {
     .....
  }
}
tt_content.uploads.override.if {
isInList.field = layout
value = 1
}
But it didn't work. What am I doing wrong?