Almost all memory is used up by files symply classified as "Other". How do I fix it?
.thumbdata is nothing but the thumbnails of the images in your gallery. They are cached into the .thumbdata file.

Other than that, most of the space will be occupied by Apps installed in your phone.

Try clearing the cache of every app that you use and uninstall the apps that you don't need.

Try using ES File Explorer to analyse the files in your phone and get a detailed view of large files, duplicate files, junk files, etc.