I would like to attach a txt file to an email and I want to be able to dynamically control the content of the txt file before attaching it. I am wanting to do this in an outlook add-in so am using office js. I have seen this doco here.
The addFileAttachmentAsync method only accepts a uri so wasn't sure if I could use this for my purposes unless I store the basic empty file somewhere and can somehow manipulate that file before attaching it.
I have seen also that there is a addFileAttachmentFromBase64Async method, which seems more useful but it needs api requirement set 1.8 which I think will preclude a lot of my users from being eligible to use it.
I'd be grateful for any suggestions.
The attachment APIs you mentioned (addFileAttachmentAsync and addFileAttachmentFromBase64Async) do not modify the content of the attachment. To achieve your scenario, you would have to create a text file and use one of the attachment APIs to attach it.
Currently, the feature to modify an attachment from an Outlook add-in is not a part of the product. We track Outlook add-in feature requests on our user-voice page. Please add your request there. Feature requests on user-voice are considered, when we go through our planning process.