Fix for #307.
Fix for #307.

Without explicitly specifying "knownfiles", only the built-in list from the mimetypes module is used, not the system defaults, and I guess `text/csv` is not in the built-in list.

maxalbert/sumatra,babsey/sumatra,apdavison/sumatra,open-research/sumatra,open-research/sumatra,open-research/sumatra,open-research/sumatra,babsey/sumatra,maxalbert/sumatra,apdavison/sumatra,maxalbert/sumatra,apdavison/sumatra,babsey/sumatra,babsey/sumatra,dpad/sumatra,dpad/sumatra,open-research/sumatra,maxalbert/sumatra,apdavison/sumatra,apdavison/sumatra,maxalbert/sumatra,babsey/sumatra,dpad/sumatra,dpad/sumatra,dpad/sumatra