Improve testcase handling in Bugzilla Template

[pyoor]

• Add a {testcase} variable to handlebars to automatically insert the testcase filename.
• The filename field should allow setting a prefix and optional suffix. The suffix should be inferred from the stored testcase file extension.
• Automatically set the mime-type when attaching the testcase.

[jschwartzentruber]

See https://github.com/MozillaSecurity/FuzzManager-dev/issues/19

[pyoor]

The testcase filename is prepopulated with l. Entering a value here (foo.bar) results in a testcase uploaded with foo.bar.undefined.