Where it says "Select file", you still need to select a file. That should be the file you're trying to import (e.g. an excel or csv file).
Detailed instructions are here in the documentation
Same issue. Dropdown does not provide anything else than "Select File".
You need to have an upload document parameter using the System.FileDocument
I create this first it then goes to a page for the user to upload the file.
Then the microflow for importing uses this upload file parameter.
Shout if I have not been clear.
Thanks