Apex Toolkit Document.fromFiles(ContentVersion) - converting Attachment to ContentVersion?

Greetings - long time lurker, first time poster :)

I’m writing some apex that creates / sends an envelope from an Opportunity, and it’s my understanding that I need to convert an Attachment SObject to a ContentVersion object in order to add it to the envelope.  I’ve created a method that looks like it should work, however when I try to send the envelope I get 

dfsle.ValidationException: The envelope is invalid.
File type is unsupported.

Here’s my code snippet:

List<dfsle.Document> docList = new List<dfsle.Document>{};
ContentVersion cVersion = new ContentVersion();
Attachment attach = [
    SELECT Id, Name, Body, ContentType, OwnerId, ParentId 
    FROM Attachment 
    WHERE Id =: attachmentId 
    LIMIT 1
];
if(attach != null){
    cVersion.ContentLocation = 'S'; //S-Document is in Salesforce
    cVersion.PathOnClient = attach.Name;    //File name with extention
    cVersion.Origin = 'H';  //C-Content Origin. H-Chatter Origin.
    cVersion.OwnerId = attach.OwnerId;  //Owner of the file
    cVersion.Title = attach.Name;   //Name of the file
    cVersion.VersionData = attach.Body; //File content
    cVersion.FirstPublishLocationId = attach.ParentId;
    insert cVersion;
    cVersion = [
        SELECT Id, ContentDocumentId, ContentLocation, PathOnClient, Origin, OwnerId, Title, VersionData, FirstPublishLocationId
        FROM ContentVersion 
        WHERE Id = :cVersion.Id
    ];
}
docList.add(dfsle.Document.fromFile(cVersion));
myEnvelope = myEnvelope.withDocuments(docList);

Not sure what I’m missing here.

Has anyone ever had success adding Attachment records to envelopes this way?  Is there a better approach?

Thanks in advance!