Storing a file is not much different than storing a string or a numerical value, although you'll need to figure out how you want to implement it.
For the most part, you're going to be passing an object stream when you are inserting the row. The object stream represents the file data. If you're holding the data in memory, you can use the ByteArrayOutputStream from the data to stream it. If the file is on the filesystem, you can just use a FileOutputStream.
The fun part in the portlet world is the actual upload of a file. In our environment, we use a servlet for the file uploads. The servlet receives the file and the 'attributes' and will store the file in the database or on the filesystem, depending upon requirements. The fronting portlet just provides the form and framework for initiating the upload through the servlet itself.
