Upload .gdoc file in file upload field

You'd need to make sure you're trying to upload an actual file that's resident on your system.  I'm not sure but I think "GDOC" files might not really be a "file" at all, but instead, something like a pointer to an online google doc URL.  You should be able to download the doc in Word DOCX format then try uploading it to Appian.

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There isn't a direct way to upload files from the Google Drive client to Appian. You'll need to download the file from Google Drive to your local system first, and then upload it to Appian.(In Simple word offline file should be present in system)