基本自訂 > User Interface Customization > Gathering the Data for the UI > File Download > Limitations
  
Limitations
This practice is limited to content types that web browsers cannot display. It cannot be used for text, HTML, XML, or common image formats such as PNG, JPEG, or GIF.
The downloadURL function works by setting the source of a hidden <iframe> to the specified URL. The browser will render text, HTML, images, etc. in the <iframe>, rather than initiating a download.
File Download