  The recent inference of sulphur dioxide (SO$_2$) in the atmosphere of the hot
($\sim$1100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST
observations suggests that photochemistry is a key process in high temperature
exoplanet atmospheres. This is due to the low ($<$1 ppb) abundance of SO$_2$
under thermochemical equilibrium, compared to that produced from the
photochemistry of H$_2$O and H$_2$S (1-10 ppm). However, the SO$_2$ inference
was made from a single, small molecular feature in the transmission spectrum of
WASP-39b at 4.05 $\mu$m, and therefore the detection of other SO$_2$ absorption
bands at different wavelengths is needed to better constrain the SO$_2$
abundance. Here we report the detection of SO$_2$ spectral features at 7.7 and
8.5 $\mu$m in the 5-12 $\mu$m transmission spectrum of WASP-39b measured by the
JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS). Our
observations suggest an abundance of SO$_2$ of 0.5-25 ppm (1$\sigma$ range),
consistent with previous findings. In addition to SO$_2$, we find broad water
vapour absorption features, as well as an unexplained decrease in the transit
depth at wavelengths longer than 10 $\mu$m. Fitting the spectrum with a grid of
atmospheric forward models, we derive an atmospheric heavy element content
(metallicity) for WASP-39b of $\sim$7.1-8.0 $\times$ solar and demonstrate that
photochemistry shapes the spectra of WASP-39b across a broad wavelength range.
