That isn't the standard use of "tacit knowledge." At least it doesn't match the definition. Tacit knowledge is supposed to be about things that are hard to communicate. The standard examples are physical activities.

Maybe knowing when to pay attention to file extensions is tacit knowledge, but the list of what they mean is easy to write down, even if it is a very long list. Knowing that it valuable to know about them is probably the key that these people were missing, or perhaps they failed to accurate assess the detail and correctness of their beliefs about file types.