SNA is an old image format used on ZX Spectrum computers for storing Mirage Microdrive snapshots. ZX Spectrum was an incipient brand of home computers released in the early 1980s and discontinued a decade later. Some of its rival brands on the European market included the Commodore 64 and Amstrad CPC. SNA images are very rare at the moment, so conversion is probably necessary in case you intend to access such a file.
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