[plotters][templates] Use file field property to read content

Plotters have properties related to their file fields
that does the proper handling for reading their content
use that.
1 job for fix_template_filefield_reading in 15 minutes and 19 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build
passed #208630
docker-build
build_linux_36

00:15:19

65.0%