XWIKI-7469: Display attachment size in the attachment index

* Add translation key
* Improve existing translation.

xwiki-platform-core/xwiki-platform-oldcore/src/main/resources/ApplicationResources.properties

@@ -2809,11 +2809,12 @@ platform.index._actions=Actions
 platform.index.emptyvalue=
 
 ### Livetable Column Labels (translationPrefix == "platform.index.attachments.")
-platform.index.attachments.filename=Filename
+platform.index.attachments.filename=Name
 platform.index.attachments.doc.fullName=Location
 platform.index.attachments.date=Date
 platform.index.attachments.author=Author
 platform.index.attachments.mimeType=Type
+platform.index.attachments.filesize=Size
 platform.index.attachments.emptyvalue=
 
 platform.index.documentsTrash=Deleted Pages