Attach large files to Outlook messages or events

Using the Microsoft Graph API, you can attach files up to 150 MB to an Outlook message or event item. Depending on the file size, choose one of two ways to attach the file:

  • If the file size is under 3 MB, do a single POST on the attachments navigation property of the Outlook item; see how to do this for a message or for an event. The successful POST response includes the ID of the file attachment.
  • If the file size is between 3MB and 150MB, create an upload session, and iteratively use PUT to upload ranges of bytes of the file until you have uploaded the entire file. A header in the final successful PUT response includes a URL with the attachment ID.

To attach multiple files to a message, choose the approach for each file based on its file size and attach the files individually.

This article illustrates the second approach step by step, creating and using an upload session to add a large file attachment (of size over 3MB) to an Outlook item. Each step shows the corresponding code for a message and for an event. Upon successfully uploading the entire file, the article shows getting a response header that contains an ID for the file attachment, and then using that attachment ID to get the raw attachment content or attachment metadata.

Important

Be aware of a known issue if you're attaching large files to a message or event in a shared or delegated mailbox.

Step 1: Create an upload session

Create an upload session to attach a file to a message or event. Specify the file in the input parameter AttachmentItem.

A successful operation returns HTTP 201 Created and a new uploadSession instance, which contains an opaque URL that you can use in subsequent PUT operations to upload portions of the file. The uploadSession provides a temporary storage location where the bytes of the file are saved until you have uploaded the complete file.

Make sure to request Mail.ReadWrite permission to create the uploadSession for a message, and Calendars.ReadWrite for an event. The opaque URL, returned in the uploadUrl property of the new uploadSession, is pre-authenticated and contains the appropriate authorization token for subsequent PUT queries in the https://outlook.office.com domain. That token expires by expirationDateTime. Do not customize this URL for the PUT operations.

The uploadSession object in the response also includes the nextExpectedRanges property, which indicates the initial upload starting location should be byte 0.

Example: create an upload session for a message

Request

POST https://graph.microsoft.com/v1.0/me/messages/AAMkADI5MAAIT3drCAAA=/attachments/createUploadSession
Content-type: application/json

{
  "AttachmentItem": {
    "attachmentType": "file",
    "name": "flower",
    "size": 3483322
  }
}

Response

The following example response shows the uploadSession resource returned for the message.

HTTP/1.1 201 Created
Content-type: application/json

{
    "@odata.context": "https://graph.microsoft.com/v1.0/$metadata#microsoft.graph.uploadSession",
    "uploadUrl": "https://outlook.office.com/api/v2.0/Users('a8e8e219-4931-95c1-b73d-62626fd79c32@72aa88bf-76f0-494f-91ab-2d7cd730db47')/Messages('AAMkADI5MAAIT3drCAAA=')/AttachmentSessions('AAMkADI5MAAIT3k0tAAA=')?authtoken=eyJhbGciOiJSUzI1NiIsImtpZCI6IktmYUNIUlN6bllHMmNI",
    "expirationDateTime": "2019-09-25T01:09:30.7671707Z",
    "nextExpectedRanges": [
        "0-"
    ]
}

Example: create an upload session for an event

Request

POST https://graph.microsoft.com/v1.0/me/events/AAMkADU5CCmSAAA=/attachments/createUploadSession
Content-type: application/json

{
  "AttachmentItem": {
    "attachmentType": "file",
    "name": "flower",
    "size": 3483322
  }
}

Response

The following example response shows the uploadSession resource returned for the event.

HTTP/1.1 201 Created
Content-type: application/json

{
    "@odata.context": "https://graph.microsoft.com/v1.0/$metadata#microsoft.graph.uploadSession",
    "uploadUrl": "https://outlook.office.com/api/v2.0/Users('d3b9214b-dd8b-441d-b7dc-c446c9fa0e69@98a79ebe-74bf-4e07-a017-7b410848cb32')/Events('AAMkADU5CCmSAAA=')/AttachmentSessions('AAMkADU5RpAACJlCs8AAA=')?authtoken=eyJhbGciOiJSUzI1NiIsImtpZCI6IktmYUNIBtw",
    "expirationDateTime": "2020-02-22T02:46:56.7410786Z",
    "nextExpectedRanges": [
        "0-"
    ]
}

Step 2: Use the upload session to upload a range of bytes of the file

To upload the file, or a portion of the file, make a PUT request to the URL returned in step 1 in the uploadUrl property of the uploadSession resource. You can upload the entire file, or split the file into multiple byte ranges. For better performance, keep each byte range less than 4 MB.

Specify request headers and request body as described below.

Request headers

Name Type Description
Content-Length Int32 The number of bytes being uploaded in this operation. For better performance, keep the upper limit of the number of bytes for each PUT operation to 4 MB. Required.
Content-Range String The 0-based byte range of the file being uploaded in this operation, expressed in the format bytes {start}-{end}/{total}. Required.
Content-Type String The MIME type. Specify application/octet-stream. Required.

Do not specify an Authorization request header. The PUT query uses a pre-authenticated URL from the uploadUrl property, that allows access to the https://outlook.office.com domain.

Request body

Specify the actual bytes of the file to be attached, that are in the location range specified by the Content-Range request header.

Response

A successful upload returns HTTP 200 OK and an uploadSession object. Note the following in the response object:

  • The ExpirationDateTime property indicates the expiration date/time for the auth token embedded in the uploadUrl property value. This expiration date/time remains the same as returned by the initial uploadSession in step 1.
  • The NextExpectedRanges specifies the next byte location to start uploading from, for example, "NextExpectedRanges":["2097152"]. You must upload bytes in a file in order.
  • The uploadUrl property is not explicitly returned, because all PUT operations of an upload session use the same URL returned when creating the session (step 1).

Example: first upload to the message

Request

PUT https://outlook.office.com/api/v2.0/Users('a8e8e219-4931-95c1-b73d-62626fd79c32@72aa88bf-76f0-494f-91ab-2d7cd730db47')/Messages('AAMkADI5MAAIT3drCAAA=')/AttachmentSessions('AAMkADI5MAAIT3k0tAAA=')?authtoken=eyJhbGciOiJSUzI1NiIsImtpZCI6IktmYUNIUlN6bllHMmNI
Content-Type: application/octet-stream
Content-Length: 2097152
Content-Range: bytes 0-2097151/3483322

{
  <bytes 0-2097151 of the file to be attached, in binary format>
}

Response

The following example response shows in the NextExpectedRanges property the start of the next byte range that the server expects.

HTTP/1.1 200 OK
Content-type: application/json

{
  "@odata.context":"https://outlook.office.com/api/v2.0/$metadata#Users('a8e8e219-4931-95c1-b73d-62626fd79c32%4072aa88bf-76f0-494f-91ab-2d7cd730db47')/Messages('AAMkADI5MAAIT3drCAAA%3D')/AttachmentSessions/$entity",
  "ExpirationDateTime":"2019-09-25T01:09:30.7671707Z",
  "NextExpectedRanges":["2097152"]
}

Example: first upload to the event

Request

PUT https://outlook.office.com/api/v2.0/Users('d3b9214b-dd8b-441d-b7dc-c446c9fa0e69@98a79ebe-74bf-4e07-a017-7b410848cb32')/Events('AAMkADU5CCmSAAA=')/AttachmentSessions('AAMkADU5RpAACJlCs8AAA=')?authtoken=eyJhbGciOiJSUzI1NiIsImtpZCI6IktmYUNIBtw
Content-Type: application/octet-stream
Content-Length: 2097152
Content-Range: bytes 0-2097151/3483322

{
  <bytes 0-2097151 of the file to be attached, in binary format>
}

Response

The following example response shows in the NextExpectedRanges property the start of the next byte range that the server expects.

HTTP/1.1 200 OK
Content-type: application/json

{
    "@odata.context":"https://outlook.office.com/api/v2.0/$metadata#Users('d3b9214b-dd8b-441d-b7dc-c446c9fa0e69%4098a79ebe-74bf-4e07-a017-7b410848cb32')/Events('AAMkADU5CCmSAAA%3D')/AttachmentSessions/$entity",
    "ExpirationDateTime":"2020-02-22T02:46:56.7410786Z",
    "NextExpectedRanges":["2097152"]
}

Step 3: Continue uploading byte ranges until the entire file has been uploaded

Following the initial upload in step 2, continue to upload the remaining portion of the file, using a similar PUT request as described in step 2, before you reach the expiration date/time for the session. Use the NextExpectedRanges collection to determine where to start the next byte range to upload. You may see multiple ranges specified, indicating parts of the file that the server has not yet received. This is useful if you need to resume a transfer that was interrupted and your client is unsure of the state on the service.

Once the last byte of the file has been successfully uploaded, the final PUT operation returns HTTP 201 Created and a Location header that indicates the URL to the file attachment in the https://outlook.office.com domain. You can get the attachment ID from the URL and save it for later use. Depending on your scenario, you can use that ID to get the metadata of the attachment, or remove the attachment from the Outlook item using the Microsoft Graph endpoint.

The following examples show uploading the last byte range of the file to the message and to the event in the preceding steps.

Example: final upload to the message

Request

PUT https://outlook.office.com/api/v2.0/Users('a8e8e219-4931-95c1-b73d-62626fd79c32@72aa88bf-76f0-494f-91ab-2d7cd730db47')/Messages('AAMkADI5MAAIT3drCAAA=')/AttachmentSessions('AAMkADI5MAAIT3k0tAAA=')?authtoken=eyJhbGciOiJSUzI1NiIsImtpZCI6IktmYUNIUlN6bllHMmNI
Content-Type: application/octet-stream
Content-Length: 1386170
Content-Range: bytes 2097152-3483321/3483322

{
  <bytes 2097152-3483321 of the file to be attached, in binary format>
}

Response

The following example response shows a Location response header from which you can save the attachment ID (AAMkADI5MAAIT3drCAAABEgAQANAqbAe7qaROhYdTnUQwXm0=) for later use.

HTTP/1.1 201 Created

Location: https://outlook.office.com/api/v2.0/Users('a8e8e219-4931-95c1-b73d-62626fd79c32@72aa88bf-76f0-494f-91ab-2d7cd730db47')/Messages('AAMkADI5MAAIT3drCAAA=')/Attachments('AAMkADI5MAAIT3drCAAABEgAQANAqbAe7qaROhYdTnUQwXm0=')
Content-Length: 0

Example: final upload to the event

Request

PUT https://outlook.office.com/api/v2.0/Users('d3b9214b-dd8b-441d-b7dc-c446c9fa0e69@98a79ebe-74bf-4e07-a017-7b410848cb32')/Events('AAMkADU5CCmSAAA=')/AttachmentSessions('AAMkADU5RpAACJlCs8AAA=')?authtoken=eyJhbGciOiJSUzI1NiIsImtpZCI6IktmYUNIBtw
Content-Type: application/octet-stream
Content-Length: 1386170
Content-Range: bytes 2097152-3483321/3483322

{
  <bytes 2097152-3483321 of the file to be attached, in binary format>
}

Response

The following example response shows a Location response header from which you can save the attachment ID (AAMkADU5CCmSAAANZAlYPeyQByv7Y=) for later use.

HTTP/1.1 201 Created

Location: https://outlook.office.com/api/v2.0/Users('d3b9214b-dd8b-441d-b7dc-c446c9fa0e69@98a79ebe-74bf-4e07-a017-7b410848cb32')/Events('AAMkADU5CCmSAAA=')/Attachments('AAMkADU5CCmSAAANZAlYPeyQByv7Y=')
Content-Length: 0

Step 4 (optional): Get the file attachment from the Outlook item

As always, getting an attachment from an Outlook item is not technically limited by attachment size.

However, getting a large file attachment in base64-encoded format affects API performance. If you expect a large attachment:

Example: Get the raw file attached to the event

Following the event example and using the attachment ID returned in the Location header of the previous step, the next example request shows using a $value parameter to get the attachment raw content data.

Request

GET https://graph.microsoft.com/v1.0/Users('d3b9214b-dd8b-441d-b7dc-c446c9fa0e69@98a79ebe-74bf-4e07-a017-7b410848cb32')/Events('AAMkADU5CCmSAAA=')/Attachments('AAMkADU5CCmSAAANZAlYPeyQByv7Y=')/$value

Response

HTTP/1.1 200 OK
content-length: 3483322
Content-type: image/jpeg

{Raw bytes of the file}

Example: Get the metadata of the file attached to the message

Following the message example, the next example request shows using a $select parameter to get some of the metadata of a file attachment on a message, excluding contentBytes.

Request

GET https://graph.microsoft.com/api/v1.0/Users('a8e8e219-4931-95c1-b73d-62626fd79c32@72aa88bf-76f0-494f-91ab-2d7cd730db47')/Messages('AAMkADI5MAAIT3drCAAA=')/Attachments('AAMkADI5MAAIT3drCAAABEgAQANAqbAe7qaROhYdTnUQwXm0=')?$select=lastModifiedDateTime,name,contentType,size,isInline

Response

HTTP/1.1 200 OK
Content-type: application/json

{
    "@odata.context": "https://graph.microsoft.com/v1.0/$metadata#users('a8e8e219-4931-95c1-b73d-62626fd79c32%4072aa88bf-76f0-494f-91ab-2d7cd730db47')/messages('AAMkADI5MAAIT3drCAAA%3D')/attachments/$entity",
    "@odata.type": "#microsoft.graph.fileAttachment",
    "@odata.mediaContentType": "image/jpeg",
    "id": "AAMkADI5MAAIT3drCAAABEgAQANAqbAe7qaROhYdTnUQwXm0=",
    "lastModifiedDateTime": "2019-09-24T23:27:43Z",
    "name": "flower",
    "contentType": "image/jpeg",
    "size": 3640066,
    "isInline": false
}

Alternative: Cancel the upload session

At any point of time before the upload session expires, if you have to cancel the upload, you can use the same initial opaque URL to delete the upload session. A successful operation returns HTTP 204 No Content.

Example: cancel the upload session for the message

Request

DELETE https://outlook.office.com/api/v2.0/Users('a8e8e219-4931-95c1-b73d-62626fd79c32@72aa88bf-76f0-494f-91ab-2d7cd730db47')/Messages('AAMkADI5MAAIT3drCAAA=')/AttachmentSessions('AAMkADI5MAAIT3k0tAAA=')?authtoken=eyJhbGciOiJSUzI1NiIsImtpZCI6IktmYUNIUlN6bllHMmNI

Response

HTTP/1.1 204 No content

Errors

ErrorAttachmentSizeShouldNotBeLessThanMinimumSize

This error is returned when attempting to create an upload session to attach a file smaller than 3 MB. If the file size is under 3 MB, you should do a single POST on the attachments navigation property of the message or of the event. The successful POST response includes the ID of the file attached to the message.