On the client side, the client's browser must support form-based upload. Most modern browsers do, but there's no guarantee. For example,
<INPUT TYPE='file' NAME='mptest'>
<INPUT TYPE='submit' VALUE='upload'>
The input type "file" brings up a button for a file select box on the browser together with a text field that takes the file name once selected. The servlet can use the GET method parameters to decide what to do with the upload while the POST body of the request contains the file data to parse.
When the user clicks the "Upload" button, the client browser locates the local file and sends it using HTTP POST, encoded using the MIME-type multipart/form-data. When it reaches your servlet, your servlet must process the POST data in order to extract the encoded file. You can learn all about this format in RFC 1867.
Unfortunately, there is no method in the Servlet API to do this. Fortunately, there are a number of libraries available that do. Some of these assume that you will be writing the file to disk; others return the data as an InputStream.