(thing-at-point 'filename) in project-find-file is a very useful feature
that often helps to use a string under point as a partial file name
to match a project file name as substring.  So it's like 'M-.'
that navigates by file names instead of identifiers.

But the problem is that in this case it drops the current file name
as the default value that is also useful in many cases.

Fortunately, the minibuffer supports a list of default values,
like in the following patch: