The standard Web Form portlet (available here in the Liferay Marketplace) does not have the ability to manage the upload of a file.
For a recent project I was asked to create a web form that would allow you to upload files and that would send to the author of the form a confirmation email (only if the author was a portal user).
Here on GitHub you can find the source code.
You can store the uploaded file in the file system or in a Documents and Media folder.
Some thing need to be improved like the data delete and export, but if you want now a web form with file upload capability…
Probably within the next month it will be available in the Liferay Marketplace.