In Facebook Messenger, sending a photo, video, or file usually starts inside the conversation where you want to share it. Open the chat, then look for the plus icon, photo icon, camera icon, or attachment option near the message box. On a phone, you can often choose a picture or video from the device gallery, take a new photo or video, and then tap send. On a computer, you can often drag and drop a file into the chat, or use the attachment button to browse your device.
If you are sending something from a phone, it may ask for permission to access your photos, videos, or files. If that happens, allow the permission so Messenger can open your media library. Some file types or very large files may not send, depending on the app version, the device, or the recipient’s settings.
If you do not see the attachment options, make sure the app is updated and check whether the conversation is in a mode that limits attachments. The current web page for Messenger or its help section may also show the exact buttons for your device, since the layout can change over time.