Support GIF formatted file whenever darkroom's parameter is given

[baskarap]

What would you like to be added

There are few cases where we want to serve GIF to our customers, but it will not work if we enable some of the parameters we support, such as: auto=compress

Why is this needed

Currently, the behaviour is to return non successful response, leading to the client to not be able to show anything. At the very least, darkroom can just return the original file in case non jpeg/png file is being accessed with darkroom's supported parameters.