On a free account you upload three documents during the morning. In the afternoon a fourth upload is refused. What is the most likely explanation?

The principle — upload limits come in two shapes, a rate over time and a total amount stored, and free accounts meet the rate first.

Why the key is correct — OpenAI states that free users are limited to three file uploads per day. Three successful uploads followed by a refusal on the same day is that limit doing exactly what it says.

Why the others are wrong — format support does not vary by tier for common documents, and a failure arriving only after several successes points at a count rather than at the file. A conversation can hold several attachments. And storage is not the constraint, since the per-user cap is far larger than three documents.

Remember this — OpenAI's own troubleshooting guidance separates the two kinds of limit: a rolling upload rate, and shared storage caps that apply across chats, projects and custom GPT knowledge. It also notes that failed attempts can count towards the rate, and that there is currently no way to check how much of your quota remains.

Sources — OpenAI's File Uploads FAQ.