- Have a public file drop share.
- Have a file, any file, ending in either of the extensions
.partor.filepart. - Attempt to drop said file onto said public share.
- Be presented with a stern warning that this is “not an allowed filetype”.
- Wonder why.
Because those file extensions are used for S3 storage. Nextcloud stores the files locally and then forwards to S3 when the upload is complete.
Are you referring to Amazon’s cloud service? Amazon does not come anywhere close to the picture.
The .part extension is commonly seen in temporary files, e.g., those created by Firefox and other browsers while downloading. But at the end of the day it is just a five character sequence with no intrinsic meaning. In my case the files refer to machine parts.
.part files are uploaded to the S3 backend where they are assembled into one file.
There are more S3 providers then amazon.
If you do not like this answer modify the source code which you have access to and remove the restriction.
I tracked it down. In fact, it’s SFA to do with Amazonian stuff, just plain OwnNextcloud daftness of its own making.
It’s an open source project. If you don’t like something change it 
Your use case is fairly unique wouldn’t you agree?