Hello, We want to use an external document management system via dedi

AFAIK base64 is the standard used in transferring files over HTTP. But I think this DMS should have some documentation telling you how to encode files.

The specification is light and I have attached an extract of it. For now, I use the following body but nothing is decoded:

--AaB03xContent-Disposition: form-data; name="content";
Content-Type: application/octet-stream
Content-Transfer-Encoding: base64

QnVzaW5lcw==
--AaB03x-

Anything wrong?

The docs talk about MTOM and XOP. I checked wikipedia for that. It will be a bit more complex.

en.wikipedia.org/.../Message_Transmission_Optimization_Mechanism
de.wikipedia.org/.../XML-binary_Optimized_Packaging

There are 2 versions of the web service: the first one is SOAP, the other one is REST. The SOAP uses MTOM which is not supported by Appian. The other option should be a standard REST web service from what I understand.

OK, I understand. The other parameters are also sent in the same format?

Yes. Here is an example. Everything works fine but the content is not decoded:
--AaB03xContent-Disposition: form-data; name="parentId"

0b02c1818249f69f
--AaB03xContent-Disposition: form-data; name="objectName"

Test.txt
--AaB03xContent-Disposition: form-data; na...
Monday, 6:10 AM Unstarred Open Message Comment
Profile photo for Fabien Bouquignaud
Fabien Bouquignaud ...me="title"

Test
--AaB03xContent-Disposition: form-data; name="subject"

--AaB03xContent-Disposition: form-data; name="authors"

ghost
--AaB03xContent-Disposition: form-data; name="keywords"

test
--AaB03xContent-Disposition: form-data; name="isFullTextIndex"

1
--AaB03xContent-Disposition: form-data; name="docDate"

08/04/2015
--AaB03xContent-Disposition: form-data; name="content";
Content-Type: application/octet-stream
Content-Transfer-Encoding: base64

QnVzaW5lcw==
--AaB03x--

Sorry, there was an error in my previous message:
--AaB03xContent-Disposition: form-data; name="parentId"

0b02c1818249f69f
--AaB03xContent-Disposition: form-data; name="objectName"

Test.txt
--AaB03xContent-Disposition: form-data; name="title"

Test
--AaB03xContent-Disposition: form-data; name="subject"

--AaB03xContent-Disposition: form-data; name="authors"

ghost
--AaB03xContent-Disposition: form-data; name="keywords"

test
--AaB03xContent-Disposition: form-data; name="isFullTextIndex"

1
--AaB03xContent-Disposition: form-data; name="docDate"

08/04/2015
--AaB03xContent-Disposition: form-data; name="content";
Content-Type: application/octet-stream
Content-Transfer-Encoding: base64

QnVzaW5lcw==
--AaB03x--

Did you try to put the boundary (--AaB03x) and the Content-Disposition in separate lines? You also need to define the used boundary in the HTTP header. Setting the content-length could also be important. Maybe you first try it outside of Appian using cURL.

stackoverflow.com/.../example-of-multipart-form-data
www.redmine.org/.../Rest_api_with_curl

Boundary and Content-Disposition are actually in separated lines. I have attached the rule that creates the body to be more precise. Boundary and content type are also defined in the header too. I will have a look at other header parameter, maybe the encoding one and the content-length as you proposed. Unfortunately I can't use cURL as the web service is only available in a VPN between the DMS and Appian cloud.

RESTBody.txt

Looks all good for me. Sorry ... I am out of questions ... Check returned error messages, try to find an example, ask DMS supplier