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First this is mainly involved Java activity inside class Data-WorkAttach-File named pyUploadFile.

Beware if you not sure what do you want to change, don't change it.

ClipboardPage cpThread = tools.getThread().getThreadPage();
ClipboardPage cpRequestor = tools.getRequestor().getRequestorPage();
ClipboardPage cpProcess = tools.findPage("pxProcess");
String exPath= cpProcess.getString("pxServiceExportPath");
String hiddenFileName= tools.getParamValue("hiddenFileName");
String pyCategory="";

java.util.StringTokenizer st = new java.util.StringTokenizer(hiddenFileName,";");
while (st.hasMoreTokens()) {

    String currentFile =(String)st.nextToken();
    if("".equals(currentFile))
        continue;
    exPath= cpProcess.getString("pxServiceExportPath");
    int indexOfSeparator = currentFile.lastIndexOf("\\");
    if(indexOfSeparator > 0){    
        currentFile=currentFile.substring(indexOfSeparator +1);
    }
    else{
        indexOfSeparator = currentFile.lastIndexOf("/");
        currentFile=currentFile.substring(indexOfSeparator +1);
    } 
    exPath+=currentFile;

 

    //Reading Uploaded file data
    try 
    {

        //String strFileName = cpRequestor.getString("pyFileUpload");
        String strFileName = exPath;
        cpRequestor.putString("pyFileUpload",exPath);
        
        int lio = exPath.lastIndexOf("/") + 1;
        String fName = StringUtils.encodeCrossOSString(exPath.substring(lio, exPath.length()));
        PRFile prfUploaded = new PRFile(exPath.substring(0, lio).concat(fName ));
        String uploadStatus="";

        ClipboardProperty pxAttachName=myStepPage.getProperty(".pxAttachName");

        if( "".equals(currentFile) )
        {
            uploadStatus= "Please select a file to upload";    

        }
        else if (!prfUploaded.exists()) {

            prfUploaded = new PRFile(exPath);
            if (!prfUploaded .exists())
            {
                uploadStatus= "File " + currentFile + " doesn't exist";
            }

        } else if (prfUploaded.length() == 0) {
            uploadStatus= "File " + currentFile + " doesn't exist or empty";

        }else if (!prfUploaded.isFile()) {

            uploadStatus= "File " + currentFile + " is not a file.";    

        } else if (!prfUploaded.canRead()) {

            uploadStatus= "File " + currentFile + " is unreadable; check the file permissions.";

        }  else {
            java.util.HashMap hmFileInfo = pega_rules_utilities.uploadFile(false);

            ClipboardPage files = tools.findPage("dragDropFileUpload",false);
            ClipboardProperty filesList = files.getProperty("pxResults");

            ClipboardPage FilePage =  tools.createPage("Embed-DragDropFile","UploadedFilePage");

            /* If uploadFile is fixed to populate the correct fileName and ext  when there are more than one dot in the fileName, We can use the hasmap provided by the uploadFile API */
            String file = currentFile; /* BUG-194578 :Do not use the encoded file name /prfUploaded.getName(); */
            String filetype = file.substring(file.lastIndexOf('.') == -1 ? file.length() : file.lastIndexOf('.')+1,file.length());//String)(hmFileInfo.get("FileExtension"));
            String fileName=file.substring(0,file.lastIndexOf('.')==-1?file.length():file.lastIndexOf('.'));
            String pyTruncatedFileName = fileName.length()>59 ? fileName.substring(0,59) : fileName;

            FilePage.putString("pyFileSource",(String)(hmFileInfo.get("FileData")));
            if(filetype.equals("")){
                            FilePage.putString("pyFileName",pyTruncatedFileName);
            }

            else{            
            FilePage.putString("pyFileName",pyTruncatedFileName+"."+filetype);
            }
            FilePage.putString("pyFileMimeType",filetype);
            FilePage.putString("pyNote",pyTruncatedFileName);
            pyCategory=myStepPage.getString("pyCategory");
            
            if("".equals(pyCategory)){
                FilePage.putString("pyCategory","File");
            }else{
                FilePage.putString("pyCategory",pyCategory);
            }
            myStepPage.putString("pyNote","");
            filesList.add(FilePage);
            FilePage.removeFromClipboard();
            tools.putParamValue("UploadStatus","good-" + hiddenFileName);
            
        }
        if (!uploadStatus.equals("")){
            ClipboardPage headerPage = cpRequestor.getPage("pyHTTPResponseHeaders");
            headerPage.putString("pxHandler", "");
            tools.putParamValue("UploadStatus",uploadStatus);
            pxAttachName.addMessage(uploadStatus);
        }
    } 
    catch (PRRuntimeException e) {
        cpThread.putString("pxHTMLStream", e.toString());
        myStepPage.addMessage(e.toString());
    }
}

In the Java Activity, Pega make Copies from the selected files into the clipboard as a page list. This will be used in pzMultiFile path control, which triggers this activity to copy the chosen files data as a page list into clipboard.

In my case, I need to make validate within the file attachment type, so in this line:

String filetype = file.substring(file.lastIndexOf('.') == -1 ? file.length() : file.lastIndexOf('.')+1,file.length());//String)(hmFileInfo.get("FileExtension"));

You got the file name also the file type of the extension, just add another validation before this line:

//This is yours validate code (For Example: if ( filetype == '.pdf ' || filetype == ',jpeg '){ )

filesList.add(FilePage);

This will prevent the action of 'attachment file'.

Hope it will help you guys!