I'm struggling to come up with a logical explanation for this right now.
Just to confirm: Is this error happening when attempting to upload the file to a post on the forum? Or is it happening in an add-on e.g. Resource Manager? If the latter, then the RM has its own allowed extensions list. Also any other add-ons may have their own list of allowed extensions also.
If it's in a forum post it may be wise to disable all add-ons and try again, just in case there's an add-on that has started to cause a problem here.