h:commandButton is not firing action if the h:form has enctype="multipart/form-data"

Question 1

Here,

for (FileItem item : items) {
    if (!item.isFormField()) {
        files.add(item);
    }
}

you're ignoring all form fields, such as the button itself. When continuing such a request, JSF will have no clue that a button was been invoked and thus not queue any action at all.

You need to add an else to collect all form fields in a Map<String, String[]> and wrap the request with a HttpServletRequestWrapper which delegates to that map on all getParameterXxx() calls and finally continue the filter chain with the wrapped request. This way JSF will find out that the button was invoked and then queue to the proper action.

Question 2

As @BalusC instructed:

import java.io.IOException;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;

import org.apache.commons.fileupload.FileItem;
import org.apache.commons.fileupload.disk.DiskFileItemFactory;
import org.apache.commons.fileupload.servlet.ServletFileUpload;


public class UploadFilter implements Filter {
    private FilterConfig filterConfig = null;

    public void init(FilterConfig filterConfig) throws ServletException {
        this.filterConfig = filterConfig;
    }

    public void destroy() {
        this.filterConfig = null;
    }

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        if (!(request instanceof HttpServletRequest)) {
            chain.doFilter(request, response);
            return;
        }

        HttpServletRequest httpRequest = (HttpServletRequest)request;

        boolean isMultipartContent = ServletFileUpload.isMultipartContent(httpRequest);

        if (!isMultipartContent) {
            chain.doFilter(request, response);
            return;
        }        

        try {
            DiskFileItemFactory factory = new DiskFileItemFactory();
            ServletFileUpload upload = new ServletFileUpload(factory);
            upload.setHeaderEncoding("UTF-8");
            upload.setSizeMax(-1);

            List<FileItem> items = upload.parseRequest(httpRequest);
            final Map<String, String[]> parameterMap = new HashMap<String, String[]>();

            for (FileItem item : items) {
                if (item.isFormField()) {
                    processFormField(item, parameterMap);
                } else {
                    processFileField(item, httpRequest);
                }
            }

            chain.doFilter(new HttpServletRequestWrapper(httpRequest) {

                    public Map<String, String[]> getParameterMap() {
                        return parameterMap;
                    }

                    public String[] getParameterValues(String name) {
                        return (String[])parameterMap.get(name);
                    }

                    public String getParameter(String name) {
                        String[] params = getParameterValues(name);
                        if (params == null) {
                            return null;
                        }
                        return params[0];
                    }

                    public Enumeration<String> getParameterNames() {
                        return Collections.enumeration(parameterMap.keySet());
                    }
                }, response);
        } catch (Exception ex) {
            ServletException servletException = new ServletException();
            servletException.initCause(ex);
            throw servletException;
        }
    }

    private void processFormField(FileItem formField, Map<String, String[]> parameterMap) {
        String name = formField.getFieldName();
        String value = formField.getString();
        String[] values = parameterMap.get(name);

        if (values == null) {
            parameterMap.put(name, new String[] { value });
        } else {
            int length = values.length;
            String[] newValues = new String[length + 1];
            System.arraycopy(values, 0, newValues, 0, length);
            newValues[length] = value;
            parameterMap.put(name, newValues);
        }
    }

    private void processFileField(FileItem fileField, HttpServletRequest request) {
        if (request.getAttribute(fileField.getFieldName()) == null) {
            List<FileItem> fileFields = new ArrayList<FileItem>(0);
            fileFields.add(fileField);
            request.setAttribute(fileField.getFieldName(), fileFields);
        } else {
            List<FileItem> fileFields = (List<FileItem>)request.getAttribute(fileField.getFieldName());
            fileFields.add(fileField);
        }
    }
}

Since I will have multiple input file component; so I have used List to store all the file fields in the method processFileField and then have set it to the request.