Test fixtures for data files should have some kind of helpful file extension

As a "user" of the issue https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=53669&q=label%3AProj-apache-commons-imaging I was confounded by the "Reproducer Testcase" file https://oss-fuzz.com/download?testcase_id=5965016805539840 which I mistakenly took to be a zip file instead of a TIFF file.

I wish the file had a more helpful extension. If none can be computed by the tool, then I'd suggest using ".bin" to indicate this is an unknown binary file.

No file, test, or entry point is named. Start by tracing how the OSS-Fuzz testcase download at the linked URL assigns filenames, then determine whether the file type can be identified; done means recognized data receives a helpful extension and unknown binary data uses .bin.

Written by the indexing model from the issue text.