as per .pdb format version 2.3, all records must start with a HEADER field.
Actually, a valid PDB format file must start with a HEADER record. All records must start with a max 6 character long record type, i.e. HEADER, COMPND, ATOM, etc.
But is the content of the file relevant for the mime-type? I thought it had to do with the extension only (and for PDB files, the typical extensions are .pdb and .ent).