Currently https://schema.org/fileFormat "MIME format of the binary (e.g. application/zip)." is a text field associated only with the SoftwareApplication type.
I propose we broaden it to apply (potentially) to any CreativeWork. This would allow us for example to say that some particular Dataset is in application/ld+json format, or that an ImageObject was image/png.
Although the name of the property is a little inelegant (it should probably have been called "media type" initially), it does not seem a huge win to change that now. I suggest instead we write something like:
CreativeWork: fileformat(Text) - "Media type (MIME format) of the content e.g. application/zip of a SoftwareApplication binary. In cases where a CreativeWork has several media type representations, 'encoding' can be used to indicate each MediaObject alongside particular fileFormat information."