When using "Source URL" "sideloading" to upload a file using the file's URL, if the file name contains URL-encoded characters, the warning "File extension does not match MIME type." is given. Removing the URL-encoded characters from the file name allows the upload to proceed, but the checkbox "Ignore any warnings" does not work.
I discovered the bug with this file, which has an URL encoded comma %2C after the word "Park":
http://commons.wikimedia.org/wiki/File:Gyps_rueppellii_-Nairobi_National_Park,_Kenya-8-4c.jpg
I'm not sure how this should be solved, since I don't know what characters are not allowed in a file name, if any. It may depend on the system. It might be a good idea to strip out characters that are allowed in URLs, but not in file names.
Version: 1.17.x
Severity: critical