XnView
From Just Solve the File Format Problem
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XnView is an image viewer and format converter. It is closed source, but free for non-commercial use. The original XnView is only for Microsoft Windows, while XnViewMP and XnConvert are also available for MacOS and Linux.
XnView supports a very large number of image formats. But it could use better documentation about the identity of the formats it claims to support, and the level of support.
There are multiple versions of XnView. Install the "Extended" version to support the largest number of image formats, and for some reason you must enable the Tools → Options → Display all image file types option.
