diff --git a/changelogs/unreleased/fj-49802-bug-api-set-http-headers.yml b/changelogs/unreleased/fj-49802-bug-api-set-http-headers.yml
new file mode 100644
index 0000000000000000000000000000000000000000..ba61d378cda03921e5be9f80569b9563a59e691a
--- /dev/null
+++ b/changelogs/unreleased/fj-49802-bug-api-set-http-headers.yml
@@ -0,0 +1,5 @@
+---
+title: Fix bug setting http headers in Files API
+merge_request: 20938
+author:
+type: fixed
diff --git a/lib/api/helpers/headers_helpers.rb b/lib/api/helpers/headers_helpers.rb
index cde51fccc62a4ef33f5d1ef6d481659863af20b0..c9c44e3c2183d7f044803a5dc6b90aa973b3da95 100644
--- a/lib/api/helpers/headers_helpers.rb
+++ b/lib/api/helpers/headers_helpers.rb
@@ -3,7 +3,11 @@ module API
     module HeadersHelpers
       def set_http_headers(header_data)
         header_data.each do |key, value|
-          header "X-Gitlab-#{key.to_s.split('_').collect(&:capitalize).join('-')}", value
+          if value.is_a?(Enumerable)
+            raise ArgumentError.new("Header value should be a string")
+          end
+
+          header "X-Gitlab-#{key.to_s.split('_').collect(&:capitalize).join('-')}", value.to_s
         end
       end
     end
diff --git a/spec/requests/api/files_spec.rb b/spec/requests/api/files_spec.rb
index 4bc5d3ee899b9dbad453666527b9352d7d719727..0aec186f7384bd6a86090602908b9c705f03e4cb 100644
--- a/spec/requests/api/files_spec.rb
+++ b/spec/requests/api/files_spec.rb
@@ -13,6 +13,24 @@ describe API::Files do
   let(:author_email) { 'user@example.org' }
   let(:author_name) { 'John Doe' }
 
+  let(:helper) do
+    fake_class = Class.new do
+      include ::API::Helpers::HeadersHelpers
+
+      attr_reader :headers
+
+      def initialize
+        @headers = {}
+      end
+
+      def header(key, value)
+        @headers[key] = value
+      end
+    end
+
+    fake_class.new
+  end
+
   before do
     project.add_developer(user)
   end
@@ -21,6 +39,18 @@ describe API::Files do
     "/projects/#{project.id}/repository/files/#{file_path}"
   end
 
+  context 'http headers' do
+    it 'converts value into string' do
+      helper.set_http_headers(test: 1)
+
+      expect(helper.headers).to eq({ 'X-Gitlab-Test' => '1' })
+    end
+
+    it 'raises exception if value is an Enumerable' do
+      expect { helper.set_http_headers(test: [1]) }.to raise_error(ArgumentError)
+    end
+  end
+
   describe "HEAD /projects/:id/repository/files/:file_path" do
     shared_examples_for 'repository files' do
       it 'returns file attributes in headers' do