If the original file-glob string is something like:

  what\?_not\*.%
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a file-glob sees a backslash escape before the "?" and "*" (to match a literal question mark and asterisk), and treats "_" and "%" as normal (literal) characters.

For conversion to an sql "like" pattern, the "\" must be removed from in front of "?" and "*" (because these characters don't need to be escaped now) and must be added in front of each "_" and "%" (to keep them literal, as they were in the file-glob).