There is not much ruby magic in that code. This is just a naive use of data from another module. Somewhere in the Gitlab code base there is a MIME_TYPES map wi

There is not much ruby magic in that code. This is just a naive use of data from another module. Somewhere in the Gitlab code base there is a MIME_TYPES map with common extensions as the map key. No idea what it is used for but that module is very likely the target of a recent security issue. The first fix to combat the "publicly unknown" vulnerability was to prevent usernames ending with any of the keys in the MIME_TYPES map using a simple "ends_with" strings check. Of course the map keys did not have periods so the ends_with would also match "Asimov" with the "mov" suffix. The second fix in this PR is to extend the ends_with check to add an extra dot. The actual vulnerability is still unknown but I suspect it's something like an intermediate component that performs special handling based on interpreting URLs and that could bypass security/ACL checks.