Users can't send attachments to Copilot Studio agents. If they try to upload a file (including media, such as images), the agent will say:
Looks like you tried to send an attachment. Currently, I can only process text. Please try sending your message again without the attachment.
This applies to all channels, even if the channel or user-facing experience supports attachments (for example, if you're using the Direct Line API or Microsoft Teams).
Attachments can be supported if the message is sent to a skill, where the skill bot supports the processing of attachments. For more information, see Use Microsoft Bot Framework skills in Copilot Studio.