diff --git a/src_assets/common/assets/web/Apps.vue b/src_assets/common/assets/web/Apps.vue
index b179a100a2d..c4bc30c277c 100644
--- a/src_assets/common/assets/web/Apps.vue
+++ b/src_assets/common/assets/web/Apps.vue
@@ -118,6 +118,8 @@
               :aria-label="$t('_common.close')"></button>
           </div>
           <div class="modal-body">
+            <!-- Error -->
+            <div v-if="editFormError" class="alert alert-danger py-2 small">{{ editFormError }}</div>
             <!-- Application Name -->
             <div class="mb-3">
               <label for="appName" class="form-label">{{ $t('apps.app_name') }}</label>
@@ -306,7 +308,7 @@
                 <input type="text" class="form-control monospace" id="appImagePath" aria-describedby="appImagePathHelp"
                   v-model="editForm['image-path']" />
                 <button class="btn btn-secondary" type="button"
-                  @click="browseFor('file', 'file_browser.select_file', editForm['image-path'], v => editForm['image-path'] = v)">
+                  @click="browseFor('file', 'file_browser.select_file', editForm['image-path'], v => editForm['image-path'] = v, ['.png'])">
                   <folder-open :size="18" class="icon"></folder-open>
                 </button>
                 <button class="btn btn-secondary" type="button" @click="showCoverFinder">
@@ -682,6 +684,7 @@
       return {
         apps: [],
         editForm: null,
+        editFormError: "",
         detachedCmd: "",
         coverSearching: false,
         coverFinderBusy: false,
@@ -691,6 +694,7 @@
         fileBrowserType: "any",
         fileBrowserTitle: "",
         fileBrowserCallback: null,
+        fileBrowserAcceptedExtensions: null,
         fileBrowserCurrentPath: "",
         fileBrowserParentPath: "",
         fileBrowserEntries: [],
@@ -802,6 +806,7 @@
           detached: [],
           "image-path": ""
         };
+        this.editFormError = "";
         this.openEditModal();
       },
       editApp(id) {
@@ -825,6 +830,7 @@
         if (this.editForm["exit-timeout"] === undefined) {
           this.editForm["exit-timeout"] = 5;
         }
+        this.editFormError = "";
         this.openEditModal();
       },
       showDeleteModal(id) {
@@ -920,10 +926,11 @@
         })
           .finally(() => this.coverFinderBusy = false);
       },
-      browseFor(type, titleKey, startPath, callback) {
+      browseFor(type, titleKey, startPath, callback, acceptedExtensions = null) {
         this.fileBrowserType = type;
         this.fileBrowserTitle = this.$t(titleKey);
         this.fileBrowserCallback = callback;
+        this.fileBrowserAcceptedExtensions = acceptedExtensions;
         this.fileBrowserSelectedPath = startPath || '';
         this.fileBrowserTypedPath = startPath || '';
         this.fileBrowserError = '';
@@ -937,6 +944,14 @@
       fileBrowserConfirm() {
         const path = this.fileBrowserSelectedPath || this.fileBrowserTypedPath;
         if (path) {
+          if (this.fileBrowserAcceptedExtensions && this.fileBrowserType !== 'directory') {
+            const lowerPath = path.toLowerCase();
+            const isValid = this.fileBrowserAcceptedExtensions.some(ext => lowerPath.endsWith(ext.toLowerCase()));
+            if (!isValid) {
+              this.fileBrowserError = this.$t('file_browser.error_invalid_extension', { extensions: this.fileBrowserAcceptedExtensions.join(', ') });
+              return;
+            }
+          }
           if (this.fileBrowserCallback) {
             this.fileBrowserCallback(path);
             this.fileBrowserCallback = null;
@@ -997,7 +1012,17 @@
         });
       },
       save() {
+        this.editFormError = "";
         this.editForm["image-path"] = this.editForm["image-path"].toString().replaceAll('"', '');
+
+        const imagePath = this.editForm["image-path"];
+        if (imagePath && !imagePath.toLowerCase().endsWith('.png')) {
+          this.editFormError = this.$t('file_browser.error_invalid_extension', { extensions: '.png' });
+          const modalBody = this.$refs.editModal.querySelector('.modal-body');
+          if (modalBody) modalBody.scrollTop = 0;
+          return;
+        }
+
         apiFetch("./api/apps", {
           method: "POST",
           headers: {
diff --git a/src_assets/common/assets/web/public/assets/locale/en.json b/src_assets/common/assets/web/public/assets/locale/en.json
index 99d6a728e0b..030f733c94d 100644
--- a/src_assets/common/assets/web/public/assets/locale/en.json
+++ b/src_assets/common/assets/web/public/assets/locale/en.json
@@ -760,6 +760,7 @@
   },
   "file_browser": {
     "empty": "No items to display",
+    "error_invalid_extension": "Selected file must have one of the following extensions: {extensions}",
     "root": "Root",
     "select": "Select",
     "select_directory": "Select Directory",
diff --git a/tests/web/Apps.test.js b/tests/web/Apps.test.js
new file mode 100644
index 00000000000..06cb6565fcb
--- /dev/null
+++ b/tests/web/Apps.test.js
@@ -0,0 +1,174 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+vi.mock('../../src_assets/common/assets/web/Navbar.vue', () => ({
+  default: { template: '<div />' },
+}))
+vi.mock('../../src_assets/common/assets/web/fetch_utils', () => ({
+  apiFetch: vi.fn(() => Promise.resolve({ status: 200 })),
+}))
+
+import { apiFetch } from '../../src_assets/common/assets/web/fetch_utils'
+import Apps from '../../src_assets/common/assets/web/Apps.vue'
+
+beforeEach(() => vi.clearAllMocks())
+
+/**
+ * Build a minimal context object that satisfies
+ * the `this` contract of fileBrowserConfirm().
+ *
+ * @param {string} path Selected file path.
+ * @param {string[]|null} acceptedExtensions Allowed extensions or null.
+ * @param {string} type Browser type (e.g. 'file', 'directory').
+ * @returns {object} A stub context for `.call()`.
+ */
+function browserContext(path, acceptedExtensions, type = 'file') {
+  return {
+    fileBrowserSelectedPath: path,
+    fileBrowserTypedPath: '',
+    fileBrowserAcceptedExtensions: acceptedExtensions,
+    fileBrowserType: type,
+    fileBrowserError: '',
+    fileBrowserCallback: vi.fn(),
+    fileBrowserClose: vi.fn(),
+    $t: (key, _params) => key,
+  }
+}
+
+/**
+ * Build a minimal context object that satisfies
+ * the `this` contract of save().
+ *
+ * @param {string} imagePath The image-path value in the edit form.
+ * @returns {object} A stub context for `.call()`.
+ */
+function saveContext(imagePath) {
+  const modalBody = { scrollTop: 100 }
+  return {
+    editForm: { 'image-path': imagePath },
+    editFormError: '',
+    $refs: { editModal: { querySelector: () => modalBody } },
+    $t: (key, _params) => key,
+    _modalBody: modalBody,
+  }
+}
+
+describe('fileBrowserConfirm – extension validation', () => {
+  it('rejects a non-PNG file when acceptedExtensions is [".png"]', () => {
+    const ctx = browserContext('/covers/cover.jpg', ['.png'])
+    const onSelect = ctx.fileBrowserCallback
+
+    Apps.methods.fileBrowserConfirm.call(ctx)
+
+    expect(ctx.fileBrowserError).toBe('file_browser.error_invalid_extension')
+    expect(onSelect).not.toHaveBeenCalled()
+    expect(ctx.fileBrowserClose).not.toHaveBeenCalled()
+  })
+
+  it('accepts a PNG with mixed-case extension', () => {
+    const ctx = browserContext('/covers/cover.PNG', ['.png'])
+    const onSelect = ctx.fileBrowserCallback
+
+    Apps.methods.fileBrowserConfirm.call(ctx)
+
+    expect(ctx.fileBrowserError).toBe('')
+    expect(onSelect).toHaveBeenCalledWith('/covers/cover.PNG')
+    expect(ctx.fileBrowserClose).toHaveBeenCalledOnce()
+  })
+
+  it('allows any extension when acceptedExtensions is null', () => {
+    const ctx = browserContext('/output/log.txt', null)
+    const onSelect = ctx.fileBrowserCallback
+
+    Apps.methods.fileBrowserConfirm.call(ctx)
+
+    expect(ctx.fileBrowserError).toBe('')
+    expect(onSelect).toHaveBeenCalledWith('/output/log.txt')
+    expect(ctx.fileBrowserClose).toHaveBeenCalledOnce()
+  })
+
+  it('skips extension check for directory type even with acceptedExtensions', () => {
+    const ctx = browserContext('/some/directory', ['.png'], 'directory')
+    const onSelect = ctx.fileBrowserCallback
+
+    Apps.methods.fileBrowserConfirm.call(ctx)
+
+    expect(ctx.fileBrowserError).toBe('')
+    expect(onSelect).toHaveBeenCalledWith('/some/directory')
+    expect(ctx.fileBrowserClose).toHaveBeenCalledOnce()
+  })
+
+  it('does nothing when no path is selected', () => {
+    const ctx = browserContext('', ['.png'])
+    const onSelect = ctx.fileBrowserCallback
+
+    Apps.methods.fileBrowserConfirm.call(ctx)
+
+    expect(ctx.fileBrowserError).toBe('')
+    expect(onSelect).not.toHaveBeenCalled()
+    expect(ctx.fileBrowserClose).not.toHaveBeenCalled()
+  })
+
+  it('uses fileBrowserTypedPath as fallback when selectedPath is empty', () => {
+    const ctx = browserContext('', ['.png'])
+    ctx.fileBrowserTypedPath = '/covers/art.png'
+    const onSelect = ctx.fileBrowserCallback
+
+    Apps.methods.fileBrowserConfirm.call(ctx)
+
+    expect(onSelect).toHaveBeenCalledWith('/covers/art.png')
+    expect(ctx.fileBrowserClose).toHaveBeenCalledOnce()
+  })
+})
+
+describe('save – cover image validation', () => {
+  it('blocks a manually typed non-PNG image path', () => {
+    const ctx = saveContext('/covers/cover.jpg')
+
+    Apps.methods.save.call(ctx)
+
+    expect(ctx.editFormError).toBe('file_browser.error_invalid_extension')
+    expect(ctx._modalBody.scrollTop).toBe(0)
+    expect(apiFetch).not.toHaveBeenCalled()
+  })
+
+  it('blocks a .bmp image path', () => {
+    const ctx = saveContext('/covers/image.bmp')
+
+    Apps.methods.save.call(ctx)
+
+    expect(ctx.editFormError).toBe('file_browser.error_invalid_extension')
+    expect(apiFetch).not.toHaveBeenCalled()
+  })
+
+  it('accepts a valid .PNG path (mixed case) and submits', () => {
+    const ctx = saveContext('/covers/cover.PnG')
+
+    Apps.methods.save.call(ctx)
+
+    expect(ctx.editFormError).toBe('')
+    expect(apiFetch).toHaveBeenCalledOnce()
+    expect(apiFetch).toHaveBeenCalledWith(
+      './api/apps',
+      expect.objectContaining({ method: 'POST' })
+    )
+  })
+
+  it('submits when the image path is empty', () => {
+    const ctx = saveContext('')
+
+    Apps.methods.save.call(ctx)
+
+    expect(ctx.editFormError).toBe('')
+    expect(apiFetch).toHaveBeenCalledOnce()
+  })
+
+  it('strips double quotes from the image path before validating', () => {
+    const ctx = saveContext('""/covers/cover.png""')
+
+    Apps.methods.save.call(ctx)
+
+    expect(ctx.editForm['image-path']).toBe('/covers/cover.png')
+    expect(ctx.editFormError).toBe('')
+    expect(apiFetch).toHaveBeenCalledOnce()
+  })
+})