diff --git a/Sources/FreeAgentCLI/Commands/Invoice/InvoiceSendEmailCommand.swift b/Sources/FreeAgentCLI/Commands/Invoice/InvoiceSendEmailCommand.swift
index c5e8985..3fedcf2 100644
--- a/Sources/FreeAgentCLI/Commands/Invoice/InvoiceSendEmailCommand.swift
+++ b/Sources/FreeAgentCLI/Commands/Invoice/InvoiceSendEmailCommand.swift
@@ -23,15 +23,31 @@ struct InvoiceSendEmailCommand: MutatingCommand {
     @Option(name: .long, help: "Email subject")
     var subject: String?
 
+    @Option(
+        name: .customLong("attachment"),
+        help: "Path to a file to attach (pdf, png, jpg, jpeg or gif), up to 5MB. Repeat for multiple files."
+    )
+    var attachments = [String]()
+
     @Flag(help: "Print the request instead of sending it")
     var dryRun = false
 
     func perform(client: Client) async throws -> EmptyResponse {
+        let attachmentPayloads = try attachments.map { path in
+            let file = try AttachmentFile(path: path)
+            return Components.Schemas.EmailAttachmentPayload(
+                data: file.data,
+                fileName: file.fileName,
+                contentType: file.contentType
+            )
+        }
+
         let emailPayload = Components.Schemas.EmailPayload(
             body: body,
             from: from,
             subject: subject,
-            to: to
+            to: to,
+            attachments: attachmentPayloads.isEmpty ? nil : attachmentPayloads
         )
 
         let invoicePayload = Operations.SendInvoiceEmail.Input.Body.JsonPayload.InvoicePayload(
diff --git a/Tests/FreeAgentCLITests/Commands/Invoice/InvoiceSendEmailCommandTests.swift b/Tests/FreeAgentCLITests/Commands/Invoice/InvoiceSendEmailCommandTests.swift
new file mode 100644
index 0000000..1d934f4
--- /dev/null
+++ b/Tests/FreeAgentCLITests/Commands/Invoice/InvoiceSendEmailCommandTests.swift
@@ -0,0 +1,24 @@
+import Foundation
+import FreeAgentAPI
+import Mockable
+import Testing
+
+@testable import FreeAgentCLI
+
+struct InvoiceSendEmailCommandTests {
+    @Test("refuses an unsupported attachment without sending the email")
+    func unsupportedAttachment() async throws {
+        let path = FileManager.default.temporaryDirectory.appendingPathComponent("\(UUID().uuidString).txt").path
+        try Data("timesheet".utf8).write(to: URL(fileURLWithPath: path))
+
+        let transport = MockClientTransportInterface()
+        let client = Client(serverURL: Environment.sandbox.baseURL, transport: transport)
+        let command = try InvoiceSendEmailCommand.parse(["1234", "--to", "alice@example.com", "--attachment", path])
+
+        await #expect(throws: AttachmentFileError.self) {
+            try await command.perform(client: client)
+        }
+
+        verify(transport).send(.any, body: .any, baseURL: .any, operationID: .value("sendInvoiceEmail")).called(0)
+    }
+}
diff --git a/openapi/components/schemas/EmailAttachmentPayload.yaml b/openapi/components/schemas/EmailAttachmentPayload.yaml
new file mode 100644
index 0000000..983fc31
--- /dev/null
+++ b/openapi/components/schemas/EmailAttachmentPayload.yaml
@@ -0,0 +1,17 @@
+type: object
+required:
+- data
+- file_name
+- content_type
+properties:
+  data:
+    type: string
+    description: Binary data of the file encoded as base64
+    example: iVBORw0KGgo...
+  file_name:
+    type: string
+    example: timesheet.pdf
+  content_type:
+    type: string
+    description: Required, but ignored - FreeAgent detects the type from the data
+    example: application/pdf
diff --git a/openapi/components/schemas/EmailPayload.yaml b/openapi/components/schemas/EmailPayload.yaml
index de5015d..037ce63 100644
--- a/openapi/components/schemas/EmailPayload.yaml
+++ b/openapi/components/schemas/EmailPayload.yaml
@@ -11,4 +11,9 @@ properties:
     example: Testing
   to:
     type: string
-    example: Test Contact <test_contact@example.com>
+    example: test_contact@example.com
+  attachments:
+    type: array
+    description: Files of up to 5MB each
+    items:
+      $ref: '#/components/schemas/EmailAttachmentPayload'
diff --git a/openapi/openapi.yaml b/openapi/openapi.yaml
index 6043c54..6b7293c 100644
--- a/openapi/openapi.yaml
+++ b/openapi/openapi.yaml
@@ -479,6 +479,8 @@ components:
       $ref: 'components/schemas/InvoiceUpdatePayload.yaml'
     EmailPayload:
       $ref: 'components/schemas/EmailPayload.yaml'
+    EmailAttachmentPayload:
+      $ref: 'components/schemas/EmailAttachmentPayload.yaml'
     TokenRequest:
       $ref: 'components/schemas/TokenRequest.yaml'
     AuthorizationCodeGrant:
diff --git a/openapi/paths/credit_notes.yaml b/openapi/paths/credit_notes.yaml
index d27248d..312a350 100644
--- a/openapi/paths/credit_notes.yaml
+++ b/openapi/paths/credit_notes.yaml
@@ -223,7 +223,7 @@ send_email:
                     body: Please find our credit note attached
                     from: Our Company <us@company.com>
                     subject: Credit Note
-                    to: Test Contact <test_contact@example.com>
+                    to: test_contact@example.com
     responses:
       '200':
         description: Success
diff --git a/openapi/paths/estimates.yaml b/openapi/paths/estimates.yaml
index 1344ae0..ddabadc 100644
--- a/openapi/paths/estimates.yaml
+++ b/openapi/paths/estimates.yaml
@@ -214,7 +214,7 @@ send_email:
                     body: Please find our estimate attached
                     from: Our Company <us@company.com>
                     subject: Estimate
-                    to: Test Contact <test_contact@example.com>
+                    to: test_contact@example.com
     responses:
       '200':
         description: Success
diff --git a/openapi/paths/invoices.yaml b/openapi/paths/invoices.yaml
index f1b5efe..aa71cd0 100644
--- a/openapi/paths/invoices.yaml
+++ b/openapi/paths/invoices.yaml
@@ -306,7 +306,7 @@ send_email:
                     body: Bison bison bison bison bison Bison bison
                     from: Our Company <us@company.com>
                     subject: Testing
-                    to: Test Contact <test_contact@example.com>
+                    to: test_contact@example.com
     responses:
       '200':
         description: Success