The j2k.dll file is an obsolete Plugin JPEG 2000 viewer for Total Commander. Total Commander is a shareware file manager. This plugin allowed users to view jp2 and jpx files from Total Commander. Jp2 and jpx files are JPEG 2000 image files.
The JPEG format is a standard image format created by the Joint Photographic Experts Group, and JPEG 2000 is an improved version of the JPEG. This particular plugin apparently became obsolete as the format became more common and other viewers supported it.
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