add gocritic/ruleguard checks back again, cleanup code. (#13665)

- remove some duplicated code
- reported a bug, separately fixed in #13664
- using strings.ReplaceAll() when needed
- using filepath.ToSlash() use when needed
- remove all non-Go style comments from the codebase

Co-authored-by: Aditya Manthramurthy <donatello@users.noreply.github.com>

cmd/handler-utils.go

@@ -289,7 +289,7 @@ func validateFormFieldSize(ctx context.Context, formValues http.Header) error {
 
 // Extract form fields and file data from a HTTP POST Policy
 func extractPostPolicyFormValues(ctx context.Context, form *multipart.Form) (filePart io.ReadCloser, fileName string, fileSize int64, formValues http.Header, err error) {
-	/// HTML Form values
+	// HTML Form values
 	fileName = ""
 
 	// Canonicalize the form values into http.Header.