How can I select a file in pop-up window?

[JMichaelTX]

@redpiano

Inside a specific application, 'open a file' action doesn't work.

How can I select a file inside pop-up window shown as attached image file?

It depends on the app, and how you want to specify the file to open.
If you'd like to provide this info, we can be of more help.

You might also look at:
How to Select a Specific File in an Open or Save Dialog Window