No Thumbnails for .jpf and .flif File Formats

[i19K1v1XTFY8]

I am having a real problem trying to get thumbnails to display for .jpf and .flif files. - I have cleared my icon cache in Windows (7 x64) and have XnView 2.46 installed with the latest .dll from the FLIF website, but even though all other file format thumbnails seem to work just fine (including .jxr) I absolutely cannot get a thumbnail preview of .jpf and .flif even though they have been associated (both my Windows and via XnView with XnView, and separately too (so no OS confusion).

I have of course ticked the relevant boxes in XnView to embed thumbnails, too, and have tested without those boxes ticked for good measure. Additionally I have restarted the machine after clearing the icon cache and deleting all file associations to the two problem file extensions; so what have I missed, or is there simply something strange with those two file formats that prevents images with .jpf or .flif file extensions from being displayed as thumbnails?

[xnview]

and if you use the flif dll from xnview package?