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Is it possible te set Content-Type to application/octet-stream using PUTUploadTransfer?

For all files whatever their real content-type? Why do you need this? Maybe you need to setup Content-Disposition if it is to allow download with "Save As" dialog?

Yes. I need to force a "Save-As" dialog regardless of the content type. Our users will store a large variety of files including jpegs etc. that need to be downloaded and not displayed in a browser.
Our development is almost complete using the PUTUploadTransfer method. Any help you can give me would be greatly appreciated.

The right way to do so is to enable CONTENT-DISPOSITION parameter:
<PARAM NAME="param8" VALUE="s3contentdisposition">
<PARAM NAME="value8" VALUE="true">

It enables "Save-As" for all uploaded file and it allows having the correct mime-type.

Does it help?

Works perfectly. Thanks for your excellent support.