TYPO3 Core

If you are overriding the whole TS configuration under

lib.fluidContent.settings.media.popup

to use your own lightbox for click-enlarging, there is no possibility to use file reference data like e.g. title and caption in the TypoScript.
You need to set a file or file reference into the content object renderer.

Example for lightbox:

lib.fluidContent.settings.media.popup >
lib.fluidContent.settings.media.popup {
  typolink {
    parameter {
      cObject = IMG_RESOURCE
      cObject.file {
        import.data = TSFE:lastImageInfo|origFile
        width = 1140m
        height = 600m
      }
      override.field = image_link
      override.listNum.stdWrap.data = register:IMAGE_NUM_CURRENT
      override.if.isTrue.field = image_link
    }
    title.data = file:current:title
    altText.data = file:current:alternative
    ATagParams = data-toggle="lightbox" data-title="{file:current:title}" data-footer="{file:current:description}" 
    ATagParams.insertData = 1
  }
}