Apparently the issue occurs either with Apache or IIS on Windows only. You're the 4th person to reference this issue (the other 3 used IIS). I will short it out in 2.10.2 or 2.10.3, but it's not my utmost priority as 99,999% of Joomla hosting is done on Linux servers with cPanel or Plesk on them.

In the meantime, as a workaround, use the "browse server" option which opens up the K2 Media Manager. Upload attachments through that and then simply select them for usage.

a) Please don't cross post. :)

b) Try commenting out this line: github.com/getk2/k2/blob/master/administrator/components/com_k2/models/item.php#L439

Let me know.