fix lack of escaping of filename in Content-Disposition (#3556)

* fix lack of escaping of filename in Content-Disposition

* add test for Content-Disposition filename escaping process

* fix filename escape bypass problem
fix backslashes before backquotes were not properly escaped problem.

context.go

@@ -1052,11 +1052,17 @@ func (c *Context) FileFromFS(filepath string, fs http.FileSystem) {
 	http.FileServer(fs).ServeHTTP(c.Writer, c.Request)
 }
 
+var quoteEscaper = strings.NewReplacer("\\", "\\\\", `"`, "\\\"")
+
+func escapeQuotes(s string) string {
+	return quoteEscaper.Replace(s)
+}
+
 // FileAttachment writes the specified file into the body stream in an efficient way
 // On the client side, the file will typically be downloaded with the given filename
 func (c *Context) FileAttachment(filepath, filename string) {
 	if isASCII(filename) {
-		c.Writer.Header().Set("Content-Disposition", `attachment; filename="`+filename+`"`)
+		c.Writer.Header().Set("Content-Disposition", `attachment; filename="`+escapeQuotes(filename)+`"`)
 	} else {
 		c.Writer.Header().Set("Content-Disposition", `attachment; filename*=UTF-8''`+url.QueryEscape(filename))
 	}