I think that the problem is: there is no icon for those file extension. If I understand correctly, the icons are located on /typo3/sysext/frontend/Resources/Public/Icons/FileIcons/
the FSC template Uploads.html reads:
<f:if condition="{data.uploads_type} == 1">
<img src="{f:uri.resource(path: 'Icons/FileIcons/{file.extension}.gif', extensionName: 'frontend')}" alt="">
</f:if>
(and there is a similar section below for
data.uploads_type=2
)
as far as I can see there is no fallback to use default.gif for unrecognized file types.
My suggestion is to modify the template and use a set of icons of yours (I would better use CSS and a sprite).
Otherwise, if you agree, I would change this issue title to "Filetype icons - add missing common filetype icons" and tracker to "Feature"... the problem is that there will for sure be debate about what file extension should be considered "common" enough to include, I think. The fallback for unrecognized filetype icons would be a nice add, but I don't know if it would be possible in FSC.
(BTW: that would be possible using CSS)