long hexadecimal-based file names of iPhone photos

People take photos on their iPhones and send them to me via iMessage. These photos are usually received with a file name formatted like IMG_XXXX.JPG where XXXX is a 4-digit decimal number or else FullSizeRender.jpg or FullSizeRender-X.jpg where X is a decimal number of one or two digits. Which all makes sense to me.

But occasionally the file name will be a long hexadecimal number-based name in the format of

XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX.JPEG i.e. an 8-digit hex number followed by three 4-digit hex numbers followed by a 12-digit hex number, all separated by hyphens with file-type appendage of JPEG instead of JPG. Does anybody know how or why this happens? All of these files still have all of their original iPhone photo meta data.