The problem seems to be in AttachmentSelector.xml in the rendering macro definition, in #attachmentPicker_displayAttachment:

(% class="$!{cssClass}#if (!$attachment) hidden#end" %)(((#if ("$!{attachmentResource}" != '' || $forceElement)#if($withLink)[[#end[[image:$services.rendering.escape(${attachmentResource}, 'xwiki/2.1')$!{imageParams}]]#if($withLink)>>attach:$services.rendering.escape(${attachmentResource},'xwiki/2.1')||rel=lightbox]]#{end}#end)))##

When there's no defaultValue macro parameter specified and the image specified in the call to the attachment selector macro doesn't exist, then $attachmentResource is empty and since #attachmentPicker_displayAttachment is called with $forceElement set to true at the following place, the image reference is set to be "filename" which doesn't exist and cause the warnings:

  (% class="attachment-picker" %)(((##
    #attachmentPicker_displayAttachment($propValue $displayImage false true) #attachmentPicker_displayButton()##
    {{html}}<input type="hidden" name="$escapetool.xml("${classname}_${object}_${property}")" value="$escapetool.xml("${propValue}")" class="property-reference"/>{{/html}}##
  )))

What I don't understand is the intent for $forceElement and why it's always passed as true. Setting it to false fixes the issue.

Marius Dumitru Florea it seems you developed this part, do you remember why there's $forceElement and why it's always passed as true?

Thanks