Hi zugang,
I’m not aware of any method that would support that by default, but you could use an S3 bucket as the storage for your files and then, basically, the files will automatically be uploaded there in a somewhat similar file structure, although this has nothing to do with NextCloud then.
If the files need to be uploaded to NextCloud, I think you will have to script something on your own for that.
You would have to run searches for files using the API and if you found a new file, you would have to get it and store it into your NextCloud instance in a folder structure you previously defined.
Outgoing Webhooks do not react on files as far as I know, so only text messages are supported at the moment.
Would you be able to script something like that?