A browser-based email system has a thin client connected to a host mail server. The thin client implements a browser. The host mail server provides pages to the thin client, which can be rendered by the browser to present an email service to a user. The thin client is equipped to receive audio and video data and supports a user interface to facilitate capture of the audio or video data for inclusion in an email message. After a data stream is captured and stored locally at the client, the browser submits a request to the host mail server that contains a token in lieu of the data stream. The host mail server returns a response specifying a new page that contains the email message. The response also includes the token and indicates a location in the new page at which the audio or video data is to be rendered. Upon receiving the response, the browser inserts the data stream into the new page at the location and renders the new page. After the user has reviewed the email message, the user can send the email message, causing the browser to submit another request to the host mail server that contains both the email message and the audio or video data. Upon receiving this second request, the host mail server converts the email message and the audio or video data to a MIME message and forwards the MIME message to the intended recipient.