This happened to work with Marcel 1.0.2 and earlier since magic byte
sniffing sees that Illustrator files are PDFs internally, causing these
files to be treated as `application/pdf` despite having a declared
content type of `application/illustrator` and an `.ai` file extension.

Marcel 1.0.3 corrected this to the more specific `application/illustrator`
subtype of `application/pdf`, but the MuPDF previewer only accepts the
parent `application/pdf` type.

Changing it to accept PDF and any child types allows the previewer to
explicitly work with Illustrator files again, which was only a happy
accident previously.