Can I accept file uploads using the Craft Guest Entries plugin? Can I accept file uploads using the Craft Guest Entries plugin? Using input type="file" doesn't seem to be working.
I don't see why you couldn't. That is unless the plugin needs to handle file uploads as opposed to the Entry Model.

Take a look at Uploading Files from a Form. You need to make sure your form tag has enctype="multipart/form-data" and your file input has the name fields[fieldHandle]. If you are uploading multiple files, be sure to add and extra set of square braces on the end (fields[fieldHandle][]).