It seems like xffm has three different methods for determining file type.
First, when it shows the icon and builds the context menu for a file, it has a
decent way of determining what type it is just by extension. When you go to
"Properties" in the context menu, it uses some other (and I'm guessing
slower, because it only needs to work on one file at a time) method to figure
out what type it is without the correct file extension.

When you open the file you would expect the program associated with it to be
determined by the second, more accurate method. But it isn't. For this action,
as far as I can tell, xffm uses a completely different and very broken way of
guessing what type of file it is based on extension. It thinks the extension of
"Mr.Bill.jpeg" is "Bill.jpeg" instead of "jpeg". So I can't easily open
any file that has more than one dot in its name. The first method does not ever
make this mistake. xffm will show the jpeg icon for "Mr.Bill.jpeg" and even
get the "Preview this image" in the context menu. But when I double-click,
xffm is stumped.

It's very clear to me that this mysterious third method needs to be dumped,
and xffm should open a file using the same type-guessing method that it does
for the "Properties" dialog for accuracy, or perhaps the directory-listing
method for consistency.