Drag and drop should expand sequential files into a single source.

[mpkepic]

Reproduced on Windows 10; referenced in both #100 and #21; creating this as a discrete bug report.

When you specify a path containing a frame sequence on the command-line, it works, however drag and drop for a folder loads each file as a separate source.

[mpkepic]

I've reproduced the issue in-isolation; the problem shows up when I have a string with double-backslashes and a format string; it incorrectly extracts the extension.

Sadly this is an apparent bug in std::filesystem; so I'm looking at working around it in xStudio.

[mpkepic]

Latest commit on the PR should resolve this, let me know if you find any edge cases, but I can drag a folder of frames in now!