you can write an activity with java step and probably can use code similar to below

// queries the database
   String sql = "SELECT * FROM pc_data_workattach WHERE pxattachname="+"\'"+FileName+"\'";
         
          /*"+"\'"+"praxis2-1.6.2.jar"+"\'"+";"; */      /*"+"\'"+FileName+"\'";*/
         
          oLog.debug(sql);
   java.sql.PreparedStatement statement = conn.prepareStatement(sql);
   //statement.setInt(1, FileName);

   java.sql.ResultSet result = statement.executeQuery();
   if (result.next()) {

String fileName = result.getString("pxattachname");
    java.sql.Blob blob = result.getBlob("pzpvstream");
    java.io.InputStream inputStream = blob.getBinaryStream();
    int fileLength = inputStream.available();
    
              //ParseState parseState = (ParseState) tools.getStepPage().getObject("pyParseState");
    //java.io.InputStream is = parseState.getInputStream();
    byte[] buffer = new byte[inputStream.available()];
    inputStream.read(buffer);

    java.io.File targetFile = new java.io.File(FileLocation+"\\"+fileName);
    java.io.OutputStream outStream = new java.io.FileOutputStream(targetFile);
    outStream.write(buffer);

    inputStream.close();
    outStream.close();    

Also: instead of 'sendFile' - consider using the Engine Class 'PRFile' to write to the server's 'ServiceRequest' directory (etc).

There is an example here of writing (a generated QR code Image) a file on the server: https://collaborate.pega.com/question/how-can-i-put-qr-code-print-reportexpexcel-or-pdf

In your case: I *think* you might have to perform an extra step to decode base64->raw bytes - check the PRPC Engine Javadocs (Designer Studio > Resources > Engine API) for 'Base64Util'

Cheers

John

Suggestion: Try writing the file data out without using Base64Util - and then examining the contents of the file with a text editor - if you are able to share this - please attach the file to the post here and we can take a look.

Not sure you got the  solution for this or not. If not, attached is the one. Attached is the screenshot from 5.5. But its the same technique in 7.x as well.

Jave step, 

You can have DSS value  prefixed to the filename to have the generic path instead of hard coding.

java.io.OutputStream stream = new java.io.FileOutputStream("<FileName with complete path>");