This is a very confusing issue description which is caused by a confusing error message. This comment https://news.ycombinator.com/item?id=28540665 does a great job explaining the context. TLDR; you can't have a username end in .filetype because it might cause the user profile page to not load. The limitation is _not_ related to injection attack prevention, that would be concerning (bobby tables xkcd https://xkcd.com/327/ ).
And it was exacerbated by another bug which was causing the absence of a period to be ignored, so any username ending in a recognized filetype was blocked (e.g. "AsiMOV" in the example, or "MaasTIFF" in the comments).
I initially suspected that a regex was involved and someone forgot to escape the period, but it looks like that wasn't even the case -- the erroneous code was literally checking if the username ended in any recognized extension.
