diff --git a/tests/Unit/Service/TransmissionServiceTest.php b/tests/Unit/Service/TransmissionServiceTest.php
index 202240ef48..1d232abf06 100644
--- a/tests/Unit/Service/TransmissionServiceTest.php
+++ b/tests/Unit/Service/TransmissionServiceTest.php
@@ -85,50 +85,43 @@ public function testGetAttachments() {
 		$this->assertEquals($expected, $actual);
 	}
 
-	public function testHandleAttachment() {
-		$id = 1;
-		$expected = [
-			'type' => 'local',
-			'id' => $id
-		];
-		[$localAttachment, $file] = [
-			new LocalAttachment(),
-			$this->createMock(ISimpleFile::class)
-		];
-		$message = $this->createMock(Message::class);
+	public function testHandleAttachment(): void {
 		$mailAccount = new MailAccount();
 		$mailAccount->setUserId('bob');
 		$account = new Account($mailAccount);
 
+		$attachment = new LocalAttachment();
+		$attachment->setFileName('test.txt');
+		$attachment->setMimeType('text/plain');
+
+		$file = new InMemoryFile(
+			'test.txt',
+			"Hello, I'm a test file."
+		);
+
 		$this->attachmentService->expects(self::once())
 			->method('getAttachment')
-			->willReturn([$localAttachment, $file]);
+			->willReturn([$attachment, $file]);
 		$this->logger->expects(self::never())
 			->method('warning');
 
-		$this->transmissionService->handleAttachment($account, $expected);
+		$part = $this->transmissionService->handleAttachment($account, ['id' => 1, 'type' => 'local']);
+
+		$this->assertEquals('test.txt', $part->getContentTypeParameter('name'));
 	}
 
-	public function testHandleAttachmentNoId() {
-		$attachment = [[
-			'type' => 'local',
-		]];
-		$message = $this->createMock(Message::class);
-		$account = new Account(new MailAccount());
+	public function testHandleAttachmentNoId(): void {
+		$mailAccount = new MailAccount();
+		$mailAccount->setUserId('bob');
+		$account = new Account($mailAccount);
 
 		$this->logger->expects(self::once())
 			->method('warning');
 
-		$this->transmissionService->handleAttachment($account, $attachment);
+		$this->transmissionService->handleAttachment($account, ['type' => 'local']);
 	}
 
-	public function testHandleAttachmentNotFound() {
-		$attachment = [
-			'id' => 1,
-			'type' => 'local',
-		];
-
-		$message = $this->createMock(Message::class);
+	public function testHandleAttachmentNotFound(): void {
 		$mailAccount = new MailAccount();
 		$mailAccount->setUserId('bob');
 		$account = new Account($mailAccount);
@@ -139,7 +132,7 @@ public function testHandleAttachmentNotFound() {
 		$this->logger->expects(self::once())
 			->method('warning');
 
-		$this->transmissionService->handleAttachment($account, $attachment);
+		$this->transmissionService->handleAttachment($account, ['id' => 1, 'type' => 'local']);
 	}
 
 	public function testGetSignMimePart() {