Certain .DMP and .ZIP files are unable to be imported. The OK button turns gray and nothing happens after the import process.

In one example trying to upload Mac High Sierra 10.13.x into the ITMS Console, under Software Management, but the "OK" bottom to accept the package is not being enabled to be clicked.

Looking at the logs you will be able to see the following type of error:

Error during partition extraction. '.dmg' is encrypted or corrupted.
Unable to extract file 0.MBR due to UnsupportedMethod

Updating the import methodology from Java to HTML5 caused this issue with certain archive files. This is a defect.

ITMS 8.1 - 8.1 RU4 (Release Update)

ITMS 8.1 RU5 has the FIX to this issue.

Workaround:

Add some dummy file into directory and set it as installation file.

It could be a simple file, like the one bellow. (Created a Test.txt and only changed the extention to .EXE)