It's a bug with the jQuery validation. Its fixed in CB 2.0.5 nightly found below.
www.joomlapolis.com/downloads/doc_details/640-community-builder-nightly-build
The mimetype validation will sometimes fail if type is missing from the browser file array, which depending on the browser it sometimes is as it's a feature not supported on all browsers. However, the validation wasn't taking this into consideration and would lock the form since it couldn't find a mimetype, but it won't do that anymore.