We're aware of a bug where it currently won't allow you to select images/videos to upload when accessed via your gallery app. Usually, there's nothing wrong on your end with the image/video, it's an implementation on our end that's broken, and a fix is on the way.
You can still upload your content, however you will have to browse through your phone's directories/folders to successfully upload them without the warning.
If you think you've found a new issue or are experiencing a bug, please submit a ticket via the in-app feedback function or firstname.lastname@example.org.
Thanks for your understanding and help, ride safe!