Illustration metadata has incorrect mime type

[benoit74]

While https://github.com/openzim/mwoffliner/issues/1808 has added the illustration metadata, when I open an mwoffliner ZIM (https://mirror.download.kiwix.org/zim/.hidden/dev/wikipedia_fr_test_2024-01.zim) with Python code, the mime type of "Illustration_48x48@1" is "text/plain".

From my understanding, it looks like it is a real issue inside the ZIM (i.e. not Python libzim reporting an incorrect mime type when reading the ZIM) because https://dev.library.kiwix.org/raw/wikipedia_fr_test_2024-01/meta/Illustration_48x48@1 is KO while https://library.kiwix.org/raw/africanstorybook.org_mul_all_2024-01/meta/Illustration_48x48@1 is fine