【Data URL】
Data URLs are composed of four parts: a prefix (data:), a MIME type indicating the type of data, an optional base64token if non-textual, and the data itself:
The mediatype is a MIME type string, such as 'image/jpeg' for a JPEG image file. If omitted, defaults totext/plain;charset=US-ASCII
参考：https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs
