diff --git a/tools/filesystem/filesystem.go b/tools/filesystem/filesystem.go
index 2e7ad542..22c64aa3 100644
--- a/tools/filesystem/filesystem.go
+++ b/tools/filesystem/filesystem.go
@@ -196,6 +196,12 @@ var inlineServeContentTypes = []string{
 	"application/pdf", "application/x-pdf",
 }
 
+// manualExtensionContentTypes is a map of file extensions to content types.
+var manualExtensionContentTypes = map[string]string{
+	".svg": "image/svg+xml", // (see https://github.com/whatwg/mimesniff/issues/7)
+	".css": "text/css",      // (see https://github.com/gabriel-vasile/mimetype/pull/113)
+}
+
 // Serve serves the file at fileKey location to an HTTP response.
 func (s *System) Serve(response http.ResponseWriter, fileKey string, name string) error {
 	r, readErr := s.bucket.NewReader(s.ctx, fileKey, nil)
@@ -210,12 +216,11 @@ func (s *System) Serve(response http.ResponseWriter, fileKey string, name string
 		disposition = "inline"
 	}
 
-	// make an exception for svg and force a custom content type
-	// to send in the response so that it can be loaded in an img tag
-	// (see https://github.com/whatwg/mimesniff/issues/7)
+	// make an exception for specific content types and force a
+	// custom content type to send in the response so that it can be loaded directly.
 	extContentType := realContentType
-	if extContentType != "image/svg+xml" && filepath.Ext(name) == ".svg" {
-		extContentType = "image/svg+xml"
+	if ct, found := manualExtensionContentTypes[filepath.Ext(name)]; found && extContentType != ct {
+		extContentType = ct
 	}
 
 	response.Header().Set("Content-Disposition", disposition+"; filename="+name)
diff --git a/tools/filesystem/filesystem_test.go b/tools/filesystem/filesystem_test.go
index e9a6a7d4..89fa78dc 100644
--- a/tools/filesystem/filesystem_test.go
+++ b/tools/filesystem/filesystem_test.go
@@ -207,6 +207,18 @@ func TestFileSystemServe(t *testing.T) {
 				"Content-Security-Policy": "default-src 'none'; style-src 'unsafe-inline'; sandbox",
 			},
 		},
+		{
+			// css exception
+			"style.css",
+			"test_name.css",
+			false,
+			map[string]string{
+				"Content-Disposition":     "attachment; filename=test_name.css",
+				"Content-Type":            "text/css",
+				"Content-Length":          "0",
+				"Content-Security-Policy": "default-src 'none'; style-src 'unsafe-inline'; sandbox",
+			},
+		},
 	}
 
 	for _, scenario := range scenarios {
@@ -216,7 +228,7 @@ func TestFileSystemServe(t *testing.T) {
 		hasErr := err != nil
 
 		if hasErr != scenario.expectError {
-			t.Errorf("(%s) Expected hasError %v, got %v", scenario.path, scenario.expectError, hasErr)
+			t.Errorf("(%s) Expected hasError %v, got %v (%v)", scenario.path, scenario.expectError, hasErr, err)
 			continue
 		}
 
@@ -321,5 +333,11 @@ func createTestDir(t *testing.T) string {
 	}
 	file4.Close()
 
+	file5, err := os.OpenFile(filepath.Join(dir, "style.css"), os.O_WRONLY|os.O_CREATE, 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+	file5.Close()
+
 	return dir
 }