@Dragonfly from the user guide:

When I upload my image/video/gif, it doesn’t attach to the post. What’s the problem?
A: It’s likely you are uploading media over the maximum allowable sizes of 8MB for images and 40MB for audio/video, or are uploading an unsupported file type. Click Here to learn more about sharing image and video files.

https://help.counter.social/en/latest/troubleshooting-faqs.html