How to upload a file with Struts2?

I’m trying put an image in a MySQL database through tag <s:file> in jsp page (I’m using Struts 2):

<s:form action="carica" id="carica" style="display:none">
    <s:file id="carica" name="caricaimg"></s:file>
    <s:submit value="Carica" ></s:submit>
</s:form>

and in my Class I made this:

public String carica() throws SQLException, FileNotFoundException{
    Connessione();   // DB connection method
    System.out.print(caricaimg);
    File file = new File(caricaimg);
    InputStream fin = new java.io.FileInputStream(file);
    int fileLength = (int)file.length();
    PreparedStatement pstmt = con.prepareStatement("INSERT INTO Utenti (NomeImg, Immagine) VALUES (?, ?)");
    pstmt.setString(1, file.getName());
    pstmt.setBinaryStream (2, fin, fileLength);
    pstmt.executeUpdate();
    return "success";
}

Everything looks ok, but when I select an image with <s:file> it returns only the name of the selected file so when I try to put the image in DB it returns this error

HTTP Status 500 – ImgName.jpg (Unable to found selected file)

This is my struts.xml file

    <?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE struts PUBLIC
   "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN"
   "http://struts.apache.org/dtds/struts-2.0.dtd">
<struts>
   <package name="Model" extends="struts-default">
      <action name="dati" class="Model.Registrazione" method="execute">
            <result name="success">/RegistrazioneRiuscita.jsp</result>
      </action>
      <action name="login">
            <result>/Login.jsp</result>
      </action>
      <action name="acces" class="Model.Registrazione" method="accesso">
            <result name="success">/LoginRiuscito.jsp</result>
            <result name="fail">/LoginFallito.jsp</result>
      </action>
      <action name="modifica" class="Model.Registrazione" method="modifica">
            <result name="success">/ModificaRiuscita.jsp</result>
            <result name="fail">/ModificaFallita.jsp</result>
      </action>
      <action name="elimina" class="Model.Registrazione" method="elimina">
            <result name="success">/EliminatoSuccesso.jsp</result>
      </action>
      <action name="carica" class="Model.Registrazione" method="carica">
            <result name="success">/index.jsp</result>
      </action>
      </package>
</struts>

This is the part that processes the selected file as method=”POST”

<action name="carica" class="Model.Registrazione" method="carica">
    <result name="success">/index.jsp</result>
</action>