Recent observations of the solar photosphere revealed the presence of elongated filamentary bright structures inside sunspot umbrae, called umbral filaments (UFs). These features differ in morphology, magnetic configuration, and evolution from light bridges (LBs) that are usually observed to intrude in sunspots. To characterize a UF observed in the umbra of the giant leading sunspot of active region NOAA 12529, we analyze high-resolution observations taken in the photosphere with the spectropolarimeter on board the Hinode satellite and in the upper chromosphere and transition region with the IRIS telescope. The results of this analysis definitely rule out the hypothesis that the UF might be a kind of LB. In fact, we find no field-free or low-field strength region cospatial to the UF. Conversely, we recognize the presence of a strong horizontal field larger than 2500 G, a significant portion of the UF with opposite polarity with respect to the surroundings, and filaments in the upper atmospheric layers corresponding to the UF in the photosphere. These findings suggest that this structure is the photospheric manifestation of a flux rope hanging above the sunspot and forming penumbral-like filaments within the umbra via magneto-convection. This reinforces a previously proposed scenario.