streamAttachmentContent does not work for Excel attachment

[ABRAR ALI]

I am trying a script to read content of an issue attachment. The script works for CSV and TXT types, but does not work for XLSX or XLS type. 

import com.atlassian.jira.component.ComponentAccessor

import com.atlassian.jira.issue.comments.CommentManager

import com.atlassian.jira.issue.AttachmentManager

import com.atlassian.jira.issue.attachment.Attachment

import com.atlassian.jira.issue.attachment.FileAttachments

import com.atlassian.jira.util.AttachmentUtils

import org.apache.commons.io.FilenameUtils

import com.atlassian.jira.util.io.InputStreamConsumer

import org.springframework.util.StreamUtils

import java.io.FileOutputStream

import java.io.InputStream

import java.io.IOException

import groovy.util.XmlParser

import groovy.util.XmlSlurper

import com.atlassian.jira.*

def issueManager = ComponentAccessor.getIssueManager()

def aM= ComponentAccessor.getAttachmentManager()

def pM = ComponentAccessor.getAttachmentPathManager()

def attachments = aM.getAttachments(issue)

String attachmentFileText=""

if (!attachments.isEmpty()) {

for (Attachment a in attachments) {

def fileName=FilenameUtils.getBaseName(a.getFilename())

log.warn (fileName)

def fileExtension=FilenameUtils.getExtension(a.getFilename())

log.warn (fileExtension)

def attachmentFile = aM.streamAttachmentContent(a, new FileInputStreamConsumer(a.getFilename()))

log.warn("This is attachmentFile" + attachmentFile)

List attachmentFileLines = attachmentFile.text.split('\n')

def righeFile=1

log.warn("This is attachmentFileLines" + attachmentFileLines)

attachmentFileLines.each(){

singleLine->

attachmentFileText=attachmentFileText+singleLine

righeFile=righeFile+1

}

return attachmentFileLines

}

}

[ABRAR ALI]

@Boris Georgiev {Appfire}  @Ravi Sagar _Sparxsys_ @Nic Brough -Adaptavist- 

Will you be able to help me resolve this. 
I am setting this script in a scripted field and it is returning values for CSV and TXT files. Excel files seems to give a non-human readable format. I don't know how to convert it.

-Abrar