TCEForms->getIconHTML() does not resolve back paths before checking static files
TCEForms->getIconHTML() returns the HTML for a static image file or a sprite icon. When checking for a static image file, the back path is not resolved before calling is_file() so references such as "../uploads/path/to/my/file.png" will fail. This is most easily reproduced with TemplaVoila and preview icons for Data Structures and Template Objects.
(issue imported from #M16703)