You first have to fetch a list of all the images of the document with GET /v1/files/PROJECT_HASH. Then, depending on the element name/id you have, you will cross check it against your list and get the corresponding file url. The file url is from amazon aws and it contains a file extension like .jpg, .png, etc.That's your actual image name, not that random hash that figma shows in the main json block.

Context: Stack Overflow question (score 0, has accepted answer): I am trying to export images used in layers. The image fills API (GET/v1/files/:key/images) returns a map of [ imageRef , URL ] but there's no way to know the image file type. For example, the URL doesn't have the file type. This is kind of a show stopper :( Ideally, I'd like to know the original file name and type(the actual image which the designer browsed), but just the file type is OK