Copilot in Outlook can summarize email threads, draft replies, and extract action items from message bodies. However, it does not read file attachments unless you explicitly tell it to. This behavior surprises many business users who expect Copilot to scan every PDF, Word document, or Excel sheet attached to an email. The root cause is a deliberate design choice by Microsoft: Copilot processes the email body and its metadata, but attachments are treated as separate data sources that require explicit grounding. This article explains why attachments are excluded by default, how to make Copilot read them, and what limitations remain.
Key Takeaways: Why Copilot Ignores Attachments in Outlook
- Copilot prompt with the file name or “/” command: You must reference the attachment in your prompt to make Copilot read it.
- Microsoft Graph data connection settings: Attachments are not part of the default email grounding scope; admins must configure data source permissions.
- Copilot plugin for OneDrive and SharePoint: Copilot can read attachments only if they are stored in a cloud location it can access.
Why Copilot Does Not Read Attachments Automatically
Copilot in Outlook uses the Microsoft Graph API to access email content. By default, the Graph endpoint for email returns the message body, subject, sender, recipients, and timestamps. Attachments are returned as file metadata, including the file name, size, and MIME type, but the file content is not loaded into the Copilot context window. This is a deliberate design to reduce latency and prevent Copilot from processing large or irrelevant files without user consent.
Another factor is data security. Attachments may contain confidential information that the email recipient is allowed to view but that should not be processed by an AI model without explicit permission. Microsoft applies the principle of least privilege: Copilot can only read data that the user has directly invoked. This prevents accidental exposure of sensitive content when Copilot summarizes threads or drafts replies.
Finally, the Copilot context window has a token limit. Each attachment, especially large PDFs or Office documents, consumes a significant number of tokens. If Copilot loaded every attachment by default, it could quickly exceed the token budget, causing the response to be truncated or the request to fail. By requiring explicit user commands, Microsoft ensures that Copilot stays within performance boundaries.
How to Make Copilot Read Attachments in Outlook
You can force Copilot to read an attachment by referencing it directly in your prompt. The following methods work in Outlook for Windows, Outlook on the web, and Outlook for Mac.
Method 1: Reference the Attachment by Name
- Open the email with the attachment
Select the email that contains the file you want Copilot to read. The attachment must be visible in the message header. - Click the Copilot icon in the Outlook ribbon
In Outlook for Windows, click the Copilot button in the Home tab. In Outlook on the web, click the Copilot icon in the top-right corner of the reading pane. - Type a prompt that includes the file name
Write a prompt such as “Summarize the quarterly report in the attached file Q4_Report.docx” or “Extract the key figures from the Budget_2025.xlsx attachment.” Copilot will then fetch the file content from the Graph API and process it alongside the email body. - Check the Copilot response
Copilot will include information from the attachment in its reply. If the attachment is too large, Copilot may respond with a message stating it cannot read the file.
Method 2: Use the Slash Command for Attachments
Outlook on the web supports a slash command that lets you reference attachments without typing the full file name.
- Open the email and click the Copilot icon
In Outlook on the web, click the Copilot icon in the reading pane toolbar. - Type a forward slash in the prompt box
Typing “/” opens a menu of available attachments from the current email. Select the file you want Copilot to read. - Add your instruction
After selecting the attachment, complete your prompt. For example, “/Q4_Report.docx summarize the key findings.” Copilot will process the attachment content.
Method 3: Upload the Attachment to OneDrive or SharePoint
If Copilot cannot read an attachment directly, you can upload the file to a cloud location that Copilot can access.
- Save the attachment to OneDrive or SharePoint
Download the attachment from Outlook and upload it to a OneDrive folder or a SharePoint document library that you have permission to access. - Open Copilot in Outlook and reference the cloud file
Type a prompt that includes the file path or name. For example, “Summarize the document at OneDrive\Documents\Project_Plan.docx.” Copilot can read files stored in OneDrive and SharePoint because those services are part of the Microsoft Graph. - Verify Copilot access
If the file is in a shared location, ensure that Copilot has permission to read it. The file must be stored in a location that is indexed by Microsoft Graph.
If Copilot Still Does Not Read Attachments
Copilot Returns a Message That It Cannot Read the Attachment
This error occurs when the attachment file type is not supported or the file is too large. Copilot in Outlook supports common Office formats: .docx, .xlsx, .pptx, .pdf, .txt, and .rtf. Files larger than 5 MB may be rejected. Convert unsupported files to a supported format or upload them to OneDrive and reference the cloud copy.
Copilot Reads the Email Body But Ignores the Attachment
This happens when you do not explicitly name the attachment in your prompt. Copilot treats attachments as optional context. Always include the file name or use the slash command to force Copilot to read the attachment. If the attachment is a .msg file, Copilot cannot open embedded emails. Save the .msg content as a .txt or .docx file instead.
Copilot in Outlook Does Not Show the Attachment in the Slash Menu
The slash command for attachments is available only in Outlook on the web. In Outlook for Windows or Mac, you must type the file name manually. Also, the slash menu shows only attachments from the currently selected email. Attachments from previous emails in the same thread are not listed. Reference those files by their full name in your prompt.
How Copilot Attachment Behavior Differs Across Microsoft 365 Apps
| Item | Outlook | Word / Excel / PowerPoint |
|---|---|---|
| Default attachment reading | Not read unless prompted | Not applicable; files are opened directly |
| Supported attachment types | .docx, .xlsx, .pptx, .pdf, .txt, .rtf | N/A — Copilot reads the open document |
| Maximum file size | 5 MB | No size limit for the active document |
| Slash command for files | Available in Outlook on the web only | Not available |
| Cloud file fallback | Upload to OneDrive or SharePoint | Open the file from OneDrive or SharePoint |
Copilot in Outlook is the only Microsoft 365 app that separates attachment processing from the main content. In Word, Excel, and PowerPoint, Copilot reads the entire open document because the file is already in the context window. In Outlook, the email body and attachments are distinct data sources that require separate grounding.
To get the most out of Copilot in Outlook, always include the attachment file name in your prompt or use the slash command in Outlook on the web. For large or unsupported files, save them to OneDrive and reference the cloud path. This approach gives you control over what data Copilot processes while maintaining security and performance.