We deal with sensitive documents and for the sake of security, we are looking for a way to upload docs directly from Appian UI to S3 instead of temporarily saving it in Appian folder and deleting it later.
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No. The only thing I have seen is people building component plugins to do such things.
The closest thing you can achieve is to delete the uploaded document at end of the process.
You can take a look at this plugin: community.appian.com/.../appian-s3-ui-file-upload-component
any luck ? have you tried the plugin ?