(This sort of crosses the line between bug and feature, though)

Describe the bug
Filesystem providers whose factories do not implement GFileSystemProbeBytesOnly or GFileSystemProbeByteProvider are not considered when opening a filesystem via 'File > Open File System...'.

To Reproduce
Steps to reproduce the behavior:

1. Create an empty filesystem whose factory implements neither GFileSystemProbeBytesOnly nor GFileSystemProbeByteProvider and make it available to Ghidra.
2. Open a filesystem via 'File > Open File System...'
3. An error message says no filesystem provider for the file is available.

Expected behavior
If the probing providers give up, there should be a way to pick a fallback filesystem.

Environment (please complete the following information):

• OS: macOS
• Java Version: 11.0.15
• Ghidra Version: 10.2-DEV at ca60845
• Ghidra Origin: locally built.

Additional context
In cases where a custom filesystem with no particular header or other identifying characteristics to write a probe for, there is no chance for the filesystem to be traversed even if a filesystem provider is available.

I've written a proto-PR to work around that issue - which I'm having now: agatti@7bf3a0e. I'm willing to clean it up and submit it as a full-fledged PR if it's OK with you folks, but maybe you may have other ideas...