Browser caches are interesting places. Firefox, in particular, has many files all with intuitive names like 'FA509DE4d01', and no extensions. Fortunately Unix culture has a method for bringing order to this chaos: 'magic' databases, or files containing bytes found in particular types of files. These allow you to guess a file's type based on its contents. Perl provides 'magic' support in the form of File::MMagic, and the following one-liner uses it to print the guessed types of all files in the current directory.
(Update: typo, and removed useless Data::Dumper)