  The near-infrared transmission spectrum of the warm sub-Neptune exoplanet GJ
1214 b has been observed to be flat and featureless, implying a high
metallicity atmosphere with abundant aerosols. Recent JWST MIRI LRS
observations of a phase curve of GJ 1214 b showed that its transmission
spectrum is flat out into the mid-infrared. In this paper, we use the combined
near- and mid-infrared transmission spectrum of GJ 1214 b to constrain its
atmospheric composition and aerosol properties. We generate a grid of
photochemical haze models using an aerosol microphysics code for a number of
background atmospheres spanning metallicities from 100 to 1000 $\times$ solar,
as well as a steam atmosphere scenario. The flatness of the combined data set
largely rules out atmospheric metallicities $\leq$300 $\times$ solar due to
their large corresponding molecular feature amplitudes, preferring values
$\geq$1000 $\times$ solar and column haze production rates $\geq$10$^{-10}$ g
cm$^{-2}$ s$^{-1}$. The steam atmosphere scenario with similarly high haze
production rates also exhibit sufficiently small molecular features to be
consistent with the transmission spectrum. These compositions imply that
atmospheric mean molecular weights $\geq$15 g mol$^{-1}$ are needed to fit the
data. Our results suggest that haze production is highly efficient on GJ 1214 b
and could involve non-hydrocarbon, non-nitrogen haze precursors. Further
characterization of GJ 1214 b's atmosphere would likely require multiple
transits and eclipses using JWST across the near and mid-infrared, potentially
complemented by groundbased high resolution transmission spectroscopy.
