filename dialog improvements?

[JonJagger]

When typing in a filename the characters backslash and space are not allowed (space is allowed at the start and is ignored). That is, they appear on the filename but the [ok] button is disabled.
Would it be better if they were filtered out as they were pressed, viz they did not appear in the filename at all?

Note that text files can be created inside the container and round-trip back to the browser.
These files can have any legal Linux filename. Viz use any character except /
This means there is a difference between filenames you can use like this and filenames you can create directly in the browser.