<p>I am using the dojo.io.iframe.send method to send a file to my server. I want to provide a way that the user can cancel the send once it is in progress, in case it is taking too long or the user realizes she sent the wrong file.</p>

<p>I can't figure a way to do this. I could use a timeout to terminate the send if it is taking a long time (that is, the server does not respond quickly), but that is not what I want. I want to terminate any time the user makes a gesture (such as clicking a "Cancel" button.</p>

<p>Thanks!</p>

For security reasons JavaScript cannot even access the file you are sending up. All file uploads are done using a form and handled by the browser directly. To my best knowledge there is no direct interface to monitor the progress of upload, nor the way to affect it (e.g., cancel it).
Navigating away may help, but it is still up to the browser to interrupt the transfer or not — there is no specification for that.
To sum it up: not possible to do it reliable in JavaScript in the portable fashion. That means Dojo does not provide this functionality.