Please check you don't force trailing slash in your urls (Settings -> URLs and SEO) since there was bug in your hotfix.
Also, I recommend to build the url manually due to performance. You wouldn't call another method like GetFileUrl and the file name isn't mandatory:
~/getattachment/{%FileFieldThatREturnsAttachmentGUID%}/file(.aspx)
or ~/getattachment/{%FileFieldThatREturnsAttachmentGUID%}/{%DocumentName%}(.aspx)
or its ASCX version.