ALSO CHECK UPLODING AN IMAGE>

A little investigation reveals:

• This key does exist in /magnolia-ui-admincentral/src/main/resources/info/magnolia/ui/admincentral/messages_en.properties line 45

• MessagesUtil is still used extensively where a key of this name is referenced in
  /magnolia-ui-form/src/main/java/info/magnolia/ui/form/field/upload/basic/BasicUploadField.java getFileDetailSize:

label.setCaption(MessagesUtil.get(fileDetailSizeCaption, "info.magnolia.ui.admincentral.messages"));

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Im not sure why the translation works for all of the other keys in the field.