David -- Puzzling, since Wikipedia tells me, "Adobe's DNG SDK provides a software library for encoding and decoding lossless JPEG with up to 16 bits per sample."JPEG-LS (usually a .jls file) is a rather unusual format to find standalone these days, though it is used as part of the DNG and DICOM standards. LrC does not understand JPEG-LS files. There were some Photoshop plugins produced for JPEG-LS, though we are going back to the days of Photoshop CS2 or thereabouts.
Can the slide digitising service not deliver in a more standard format such as TIFF or DNG?
As I said in my initial reply, whilst JPEG-LS is used as part of the DNG and DICOM standards, it is rarely found as a standalone format, and I am fairly certain that Lightroom Classic will not accept a JPEG-LS file. Victoria's idea of asking for a sample file seems sensible.
According to Dave (see quote above), his files can be imported by Photoshop. I "own" photoshop, according to my subscription with Adobe, but I've never used it. Does anyone know if it will export files without loss in another format that can be imported by LrC? -- jclarkw