Does anyone have any idea what's causing this to show up? Given that the file extension .cdxml is a permitted file extension in the environment and the user has confirmed that the file type matches the extension.
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Appian tries to read the file you uploaded it tries to match whether contents match it's file type. Are you sure your file is cdxml type.
I found this other thread where Mike explained it very clearly (It might help you to resolve the issue)
https://community.appian.com/discussions/f/general/23862/the-file-s-extension-png-does-not-match-its-underlying-file-type
The file is of cdxml type. . Thanks for the link to Mike's thread. Seems a like a support case is needed in this case.