In the list of media files in the library, the flagged media now have an orange star in the second-last column from the right. The orange star is used to flag a media file or files to which you wish to draw a special attention, whether they are a favourite media file or files to modify or check. From the list of media files, you can click on the corresponding star to either flag or unflag the file.
NOTE: When a media's form is displayed, you can move the cursor over the gear icon in the top right of the form and click on Flag.