Steven Walter , sorry not 100% sure of your use case now (I think the wiki is no longer required?). Depending on your use case a file meta data iSheet can be very useful. I am not sure if you are familiar but this means that when a document is uploaded to the system a record in the file meta data iSheet is automatically created. At the time of the upload(via UI) you can also add questionnaire data to it like a normal iSheet. Workflows based on a file MD iSheet pushes a normal iSheet worksheet further by allowing you copy/move/delete files and add folders. This questionnaire update is handled slightly differently in the backend where its "upload file, read fileid and then hit the endpoint (sorry cant remember exact name) but its essentially GetiSheetItemIdByFileId which would then allow you to update the related isheet record. Alternatively you could go the iSheet like normal and retrieve the system id based on a unique value in the isheet (like any normal isheet)

HighQ also annouced a significant improvement to attachment columns which will enable you to allow users to use these columns to import into a designated folder in the files module (EDIT: Announced for 5.4)

The above may not be useful to you so to answer your question directly, you only need to send the API the pieces which are being updated (it will not change anything else).

I have never imported into an attachment column, however I believe you have to import the attachment into the isheet and then edit the record to reference the attachement id. This should help : https://collaborate.highq.com/sitepoint/wikiHome.action?metaData.siteID=714&metaData.wikiID=26416 https://collaborate.highq.com/sitepoint/viewWikiPage.action?metaData.siteID=714&metaData.wikiID=27275

A similiar result can be achieved using a file link column (upload into the files module, add file id reference to iSheet record)

Just seen your response, hopefully the above could be helpful a little bit!