diff --git a/Bugzilla/Template.pm b/Bugzilla/Template.pm
index adf35a64b3..ad6fdc11b6 100644
--- a/Bugzilla/Template.pm
+++ b/Bugzilla/Template.pm
@@ -97,6 +97,11 @@ sub get_format {
   $ctype =~ s/[^a-zA-Z\-]//g;
   $format =~ s/[^a-zA-Z\-]//g;
 
+  # Content types are lowercase. Normalise so that e.g. "CSV" cannot miss
+  # the contenttypes lookup or resolve to a template on a case-insensitive
+  # filesystem while bypassing callers' checks against $format->{extension}.
+  $ctype = lc $ctype;
+
   $template .= ($format ? "-$format" : "");
   $template .= ".$ctype.tmpl";
 
diff --git a/t/template-get-format-case.t b/t/template-get-format-case.t
new file mode 100644
index 0000000000..2a77e1f950
--- /dev/null
+++ b/t/template-get-format-case.t
@@ -0,0 +1,52 @@
+#!/usr/bin/env perl
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# This Source Code Form is "Incompatible With Secondary Licenses", as
+# defined by the Mozilla Public License, v. 2.0.
+
+# Bug 2081090 - get_format must treat ctype case-insensitively so that a
+# value like "CSV" cannot bypass the contenttypes lookup or callers'
+# checks against $format->{extension}.
+
+use 5.10.1;
+use strict;
+use warnings;
+
+use lib qw(. lib local/lib/perl5);
+use Test::More;
+
+BEGIN {
+  use_ok('Template');
+  use_ok('Bugzilla::Template');
+}
+
+# get_format only needs $self->context->template, so a plain Template
+# Toolkit object over the real template directory is enough. On a
+# case-sensitive filesystem this also proves the lowercase template name
+# is the one being resolved.
+my $template = bless Template->new({INCLUDE_PATH => 'template/en/default'}),
+  'Bugzilla::Template';
+
+foreach my $ctype (qw(csv CSV Csv cSv)) {
+  my $format = $template->get_format('list/list', '', $ctype);
+  is($format->{extension}, 'csv',      "ctype=$ctype: extension is csv");
+  is($format->{ctype},     'text/csv', "ctype=$ctype: content type is text/csv");
+  is($format->{template}, 'list/list.csv.tmpl',
+    "ctype=$ctype: template name is lowercase");
+}
+
+my $html = $template->get_format('list/list', '', 'HTML');
+is($html->{extension}, 'html',      'ctype=HTML: extension is html');
+is($html->{ctype},     'text/html', 'ctype=HTML: content type is text/html');
+
+my $default = $template->get_format('list/list', '', undef);
+is($default->{extension}, 'html', 'missing ctype defaults to html');
+
+my $unknown = $template->get_format('global/variables', '', 'NONE');
+is($unknown->{extension}, 'none', 'unknown ctype is lowercased');
+is($unknown->{ctype}, 'application/octet-stream',
+  'unknown ctype falls back to application/octet-stream');
+
+done_testing();