In Plotter file selector, cannot select C3D files by default

[chrisdembia]

Steps to reproduce

1. Tools > Plot.
2. Right-click "Y-Quantity".
3. Navigate to a folder containing a C3D file.

Expected result

I can select a C3D file.

Actual result

I cannot select a C3D file by default.

If I click the "File Format" dropdown and select "All Files", then I can load and plot data from a C3D file.

We should add C3D as an accepted file format for the plotter.

Environment and GUI version

Mac 4.0-2018-09-05-447b65a

[aymanhab]

@chrisdembia Is default .c3d, or .C3D will likely allow both but wanted to know if there's standard conevtion

[chrisdembia]

I'd say .c3d.

[chrisdembia]

I don't know if there is a convention specific to C3D files, just that file extensions are usually lowercase.