Test that JPEG validation is actually working

[technopagan]

While using IM's identify to check on the validity of an incoming JPEG image has the benefit of not intrudicing yet another tool & thus dependency, it is prone to return incorrect results: if a JPEG image is only partially transferred but its headers are intact, IM identify will vaildate it as a valid file.

Let's do extensive tests to make sure that no invalid files can be processed through Adept. In case IM's identify should not match our expectations, we might switch to a tool such as jpeginfo.

See:

• https://github.com/tjko/jpeginfo
• http://manpages.ubuntu.com/manpages/hardy/man1/jpeginfo.1.html