MDL-32471 introduced a function to get a file preview. As part of that, it gets the image dimensions using getimagesize(). In order to do so, it writes the image data (a string) to a file, and then calls getimagesize() against the file.

Since it was introduced, we have bumped the requirements for Moodle, and now require PHP 5.4 which introduced a getimagesizefromstring() function. This would negate the need to store the file on disk.