surely, is legal in winblows os'es too, but the question is that: the user specifies an acceptedTypes list-comma separated string inside the page in the <rich:fileUpload> component, and this must be a list of extensions without the '.' dot. 
   Inside the component reference spec. the use of dots however is not explicitly required...
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acceptedTypes

The acceptedTypes parameter defines comma separated list of file extensions accepted by component. The component does not provide any feedback when rejecting file. For introducing feedback for rejection, use ontyperejected parameter

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...so if we implicit take the assumption (as the example source code in showcase does) that we must pass a string of comma separated extensions without dot '.' prefixes, the only way the user can specify an empty file extension is when he inserts a comma followed by 0 or more spaces or vice-versa if it is the first entry in the acceptedTypes string. 
If you try to do that, the actual implementation will upload each file without care of extension (i have to look inside code deeply to understand if it's a bug or it is a sort of failed validation of acceptedTypes list behaviour, then it will disable the effects of acceptedTypes attribute at all...try it yourself!). 
So we have to modify the acceptedTypes to require the "." prefix, then the user can specify an acceptedTypes=".,.JPG,.PNG" for example...otherwise, as it is now, it make sense to reject the "noextension." files because they have and empty extension that it will represented by an empty string and the users actually cannot specify that inside the acceptedTypes because as i sayed before inserting a comma followed by an empty string will change the expected behaviour, disabling the per-extension upload filter ...WDYT?