Currently, when PyClowder2 uploads a file preview, its MIME type is obtained from the file extension. It's possible that preview files with custom / uncommon MIME types need to be uploaded from an extractor. We can support this feature by accepting an optional field for MIME type in https://opensource.ncsa.illinois.edu/bitbucket/projects/CATS/repos/pyclowder2/browse/pyclowder/files.py#191, which defaults to None. By default, files module will upload preview as is, but if this field contains a MIME type string, it will be used when creating the file preview.