Thank you very much! That does what I want and is clear to me:

use Plack::Builder;
use Plack::MIME;

my $expr = qr{\.c|\.cpp|\.csv|\.dat|\.dpl|\.gp|\.h|\.hpp|\.html|
              \.ini|\.java|\.js|makefile|\.m|\.mac|
              \.pl|\.pm|\.pod|\.py|readme|\.sch|\.sh|\.txt}xi;

builder {
  # normal behaviour outside of downloads
  enable 'Plack::Middleware::Static',
    path => qr{^/html/(?!downloads/)},
    root => '.';
    
  # plain text for non binary download files
  enable 'Plack::Middleware::Static',
    path => qr{^/html/downloads/.*($expr)$},
    root => '.',
    content_type => 'text/plain';
  
  # download only for the other download files
  enable 'Plack::Middleware::Static',
    path => qr{^/html/downloads/.*(?!$expr)$},
    root => '.',
    content_type => 'mime/type';
    
  mount "/" => $app1;
  mount "/imggenerator.pl" => $app2;
};
[download]