Add MIME type as an optional parameter to files.json REST API endpoint

When using the /rest/asset/v1/files.json endpoint, being able to use MIME type as a querying parameter would be useful, especially when dealing with Design Studio instances where foldering has not been properly used. Being able to pull all PDFs with application/pdf or PNGs with image/png would be helpful for paring down results to just certain types of files. The MIME type today is already returned when using this endpoint.