fix(attachments): don't preview attachments when viewer app is disabled

src/mixins/AttachmentMixin.js

@@ -17,6 +17,10 @@ export default {
 		},
 
 		previewableFileInfos() {
+			if (!OCA?.Viewer) {
+				return []
+			}
+
 			return this.fileInfos.filter((fileInfo) => (fileInfo.mime.startsWith('image/')
 				|| fileInfo.mime.startsWith('video/')
 				|| fileInfo.mime.startsWith('audio/')

src/tests/unit/mixins/AttachmentMixin.spec.js

@@ -0,0 +1,45 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { shallowMount } from '@vue/test-utils'
+import AttachmentMixin from '../../../mixins/AttachmentMixin.js'
+
+const attachments = [
+	{ downloadUrl: '/cat.png', fileName: 'cat.png', mime: 'image/png' },
+	{ downloadUrl: '/notes.txt', fileName: 'notes.txt', mime: 'text/plain' },
+]
+
+function mountMixin() {
+	return shallowMount({
+		mixins: [AttachmentMixin],
+		data: () => ({ attachments }),
+		render: (h) => h('div'),
+	})
+}
+
+describe('AttachmentMixin', () => {
+	afterEach(() => {
+		delete global.OCA
+	})
+
+	it('previews attachments supported by the viewer', () => {
+		global.OCA = { Viewer: { mimetypes: ['image/png'] } }
+
+		const view = mountMixin()
+
+		expect(view.vm.previewableFileInfos).toHaveLength(1)
+		expect(view.vm.canPreview(view.vm.fileInfos[0])).toBe(true)
+		expect(view.vm.canPreview(view.vm.fileInfos[1])).toBe(false)
+	})
+
+	it('previews nothing when the viewer app is disabled', () => {
+		global.OCA = {}
+
+		const view = mountMixin()
+
+		expect(view.vm.previewableFileInfos).toEqual([])
+		expect(view.vm.canPreview(view.vm.fileInfos[0])).toBe(false)
+	})
+})