Twitter announced Monday it is blocking animated PNG (APNG) files after trolls used them to attack epileptics.

APNGs are similar to animated GIFs, containing multiple images in a single file. The images, or frames, are displayed sequentially, creating the effect of an animation or video clip.

Unlike animated GIFs, however, APNGs bypassed Twitter’s safeguards that prevent animated files from auto-playing. Online trolls took advantage of this, hijacking hashtags used by the Epilepsy Foundation and displaying flashing APNGs in an effort to cause seizures.

To prevent this from happening again, Twitter has blocked APNGs from being used. Users will still be able to upload animations, but will need to use animated GIFs, as they don’t auto-play and therefore don’t post the same risk to users.

Twitter’s announcement:

“We want everyone to have a safe experience on Twitter.

APNGs were fun, but they don’t respect autoplay settings, so we’re removing the ability to add them to Tweets. This is for the safety of people with sensitivity to motion and flashing imagery, including those with epilepsy.”

— Twitter Accessibility (@TwitterA11y) December 23, 2019