Groovy Script: XmlNodePrinter adds one extra line while writing into xml file.

In that case, I would suggest the following:

Have the original file with data as below which uses property expansion as mentioned earlier, you may call it template request.

<abc-request client-application-id="JUnit client" client-domain="JUnit client Domain">
    <payload>
        <modify-UG>
            <charging-id type="msisdn">${#TestCase#MSISDN}</charging-id>
         </modify-UG>
    </payload>
</abc-request>

Before your json step, have a groovy step which does the transformation of the data and saves into a file in the temporary directory by replacing the MSISDN value. 

This way, we always only read the template file(remains same) and create a new temporary file and that is sent as attachment each time. So, no overwriting of that template file.

def tmp = System.getProperty('java.io.tmpdir')
context.testCase.setPropertyValue('FINAL_DIR', tmp)
def originalFilePath = context.expand('${#TestCase#ORIGINAL_FILE_PATH}')
def fileName = context.expand('${#TestCase#FINAL_FILE}')
def content = new File(originalFilePath).text
new File("${tmp}/${fileName}").write(context.expand(content))

Note: If you are using ReadyAPI, then you may avoid additional groovy step, instead use Events feature, TestStep.beforeSubmit and have the above script in it

In the attachment, you define the custom properties as below: