Board Thread:In-Editor Help Assistance/@comment-39228942-20191201200136/@comment-9605025-20191201200649

For that message, there are two common causes: If the latter, try logging out, closing your browser, reopen your browser, login, and retry.
 * 1) The wiki is using the abuse filter and your attempted upload has been blocked by it
 * 2) There is something wrong with your login session

If it were an issue with the file itself, you would likely be getting a different message. - Edit:

Based on this recent thread, it could also be an issue with your cookies.