>> I see the same issue on Fedora, with ImageMagic 6.9.9-19:
>>
>> # Failed test at t/img.t line 119.
>> # got: 'no image: Exception 435: unable to open image `:t/tmp/out/imgconversions/10x-redsquare.png': No such file or directory @ error/blob.c/OpenBlob/2701'
>> # expected: '10x10'
>> [...]
>>
>> So it seems, that an empty coder prefix is not accepted anymore. To me it seems that [this commit](https://github.com/ImageMagick/ImageMagick/commit/4bc9b6b) changed the behavior. Unfortunately, the commit message doens't tell us about the reasons behind. The commit is included from version 6.9.8-3 on.