{"id":25421,"date":"2026-03-18T08:28:31","date_gmt":"2026-03-18T15:28:31","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/microsoft365dev\/?p=25421"},"modified":"2026-03-18T08:28:31","modified_gmt":"2026-03-18T15:28:31","slug":"changes-to-attachment-ids-for-inline-images-in-outlook-add-ins","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/microsoft365dev\/changes-to-attachment-ids-for-inline-images-in-outlook-add-ins\/","title":{"rendered":"Changes to attachment IDs for inline images in Outlook add-ins"},"content":{"rendered":"

We\u2019d<\/span>\u00a0like to inform you about an update to how\u00a0<\/span>inline\u00a0<\/span>image attachments are handled\u00a0<\/span>when\u00a0<\/span>they\u2019re<\/span>\u00a0<\/span>programmatically<\/span>\u00a0<\/span>added to a mail item’s<\/span>\u00a0signature\u00a0<\/span>or<\/span>\u00a0body<\/span>\u00a0<\/span>in Outlook on the web and the new Outlook for Windows.<\/span>\u00a0<\/span>This\u00a0<\/span>change\u00a0<\/span>may\u00a0<\/span>impact<\/span>\u00a0your add<\/span>–<\/span>in if it\u00a0<\/span>relies\u00a0<\/span>on<\/span>\u00a0<\/span>the Exchange Web Services (<\/span>EWS<\/span>)\u00a0<\/span>ID\u00a0<\/span>that\u2019s\u00a0<\/span>returned when adding\u00a0<\/span>inline\u00a0<\/span>images<\/span>.<\/span><\/p>\n

What’s<\/span>\u00a0changing?<\/span><\/h2>\n

Previously, when\u00a0inline images\u00a0were added to\u00a0a\u00a0mail item\u2019s\u00a0signature or body\u00a0using\u00a0the\u00a0<\/span>addFileAttachmentFromBase64Async<\/code>\/addFileAttachmentAsync<\/code> method (<\/span>Message Compose<\/span><\/a>)\u00a0with\u00a0<\/span>isInline<\/code>\u00a0set\u00a0to\u00a0<\/span>true<\/strong>,\u00a0the\u00a0images\u00a0were uploaded\u00a0to the server\u202fbefore\u202fthe\u00a0call\u00a0completed.\u00a0An EWS ID was then returned in the callback function for each image (for example,\u00a0<\/span>AAkALgAAAAAAHYQDEapmEc2byACqAC\/EWg0AW4xUByFbp0CroCNphMYEEAAGhqU3LwAAARIAEACS5VsVp3akQa7\/BahqVbve<\/code>).<\/span>\u00a0<\/span><\/p>\n

Going forward,\u00a0inline images\u00a0will be\u00a0uploaded to the\u00a0server\u202fafter\u00a0the\u00a0<\/span>addFileAttachmentFromBase64Async<\/code>\/addFileAttachmentAsync<\/code> call completes and the images are added to a signature or body. Because the <\/span>addFileAttachmentFromBase64Async<\/code>\/addFileAttachmentAsync<\/code><\/span>\u00a0<\/strong>call completes\u00a0immediately\u00a0without a network call, a temporary attachment ID is\u00a0generated locally instead of an EWS ID.\u00a0The attachment ID is\u00a0prefixed\u00a0with \u201caddinId\u201d (for example,\u00a0<\/span>addinId:31c91015-920a-6fed-f663-970a0f9bc5cd<\/code>).\u00a0The\u00a0locally generated\u00a0attachment ID is only temporary\u00a0because\u00a0an EWS ID is eventually generated for the attachment once\u00a0it\u2019s\u00a0been uploaded to the server.<\/span>\u00a0<\/span><\/p>\n

The\u00a0temporary\u00a0attachment ID is fully supported\u00a0with other attachment APIs, such as\u00a0<\/span>getAttachmentsAsync<\/code>\u00a0and\u00a0<\/span>getAttachmentContentAsync<\/code>,\u00a0for the duration of the\u00a0current\u00a0compose session\u00a0(that is, while the mail item being composed is open).\u00a0This\u00a0behavior\u00a0applies even after the attachments have been uploaded and assigned an EWS ID.\u00a0Once the compose session ends, the\u00a0temporary ID can no longer be used.<\/span>\u00a0<\/span><\/p>\n

Similar to the previous approach, users won’t be able to send the mail item until all inline attachments have been uploaded to the server. If a user attempts to send a message while attachments are still being uploaded, a dialog is shown informing them that the upload is still in progress.<\/span>\u00a0<\/span><\/p>\n

How does this change affect the\u00a0<\/span>getAttachmentsAsync<\/span>\u00a0call?<\/span><\/h2>\n

When you<\/span>\u00a0call\u00a0<\/span>the\u00a0<\/span><\/span>getAttachmentsAsync<\/span><\/span><\/code>\u00a0method<\/span>\u00a0<\/span>in\u00a0<\/span><\/span>message<\/span><\/span><\/a> compose mode<\/span>\u00a0after programmatically inserting inline attachments, the\u00a0<\/span>attachment ID\u00a0<\/span>returned in the callback function varies depending on whether the attachments have\u00a0<\/span>been uploaded to the server.<\/span><\/span>\u00a0<\/span><\/p>\n