XnView is an excellent image viewing program that handles many graphics formats and allows numerous batch operations like: resizing, compressing, image format conversion, etc.
By default, it only supports a subset of the image formats it is capable of handling. When you try to open a format it doesn’t recognize (but very likely handles), you get the following message:

Unfortunately, it is not immediately obvious where this setting is (at least it wasn’t for me).
1) Click on Tools on the menu bar:

2) Select Options from the drop down menu:

3) Click on the General label in the dialog. Then select the checkbox next to the option Display all image file types. Then click on Ok to save the results. [image may be clicked for larger size]

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