what extensions are acceptable for attachments?

what extensions are acceptable for attachments?

a few days ago, I wanted to attached a file with an .html extension to a post, and the system said that extension is unacceptable..    ahhhhhhh... say what????  this is weird...

so what extensions are acceptable?  if obvious extensions like .html are not acceptable then there should be a list of acceptable extensions right next to the "attachment" link, so people don't get told after they've tried to upload a file that the extension of the file they're trying to attach is not acceptable...

thank you...