diff --git a/resources/js/pages/Acp.jsx b/resources/js/pages/Acp.jsx
index e0f8080..35c633b 100644
--- a/resources/js/pages/Acp.jsx
+++ b/resources/js/pages/Acp.jsx
@@ -1140,34 +1140,63 @@ export default function Acp({ isAdmin }) {
         setAttachmentGroupsError('')
         setAttachmentExtensionsError('')
         try {
+            const groupsByName = new Map(attachmentGroups.map((group) => [group.name, group]))
+            const extensionsByGroup = new Map()
+            attachmentExtensions.forEach((ext) => {
+                const groupId = ext.attachment_group_id ? String(ext.attachment_group_id) : 'none'
+                if (!extensionsByGroup.has(groupId)) {
+                    extensionsByGroup.set(groupId, new Set())
+                }
+                extensionsByGroup
+                    .get(groupId)
+                    .add(String(ext.extension || '').toLowerCase())
+            })
             const createdGroups = []
             const createdExtensions = []
 
             for (const group of attachmentDefaultSeed) {
-                const parent = await createAttachmentGroup({
-                    name: group.name,
-                    max_size_kb: group.max_size_kb,
-                    is_active: true,
-                })
+                let parent = groupsByName.get(group.name)
+                if (!parent) {
+                    parent = await createAttachmentGroup({
+                        name: group.name,
+                        max_size_kb: group.max_size_kb,
+                        is_active: true,
+                    })
+                    groupsByName.set(group.name, parent)
+                    createdGroups.push(parent)
+                }
                 createdGroups.push(parent)
 
                 for (const child of group.children || []) {
-                    const createdChild = await createAttachmentGroup({
-                        name: child.name,
-                        parent_id: parent.id,
-                        max_size_kb: child.max_size_kb,
-                        is_active: true,
-                    })
-                    createdGroups.push(createdChild)
+                    let createdChild = groupsByName.get(child.name)
+                    if (!createdChild) {
+                        createdChild = await createAttachmentGroup({
+                            name: child.name,
+                            parent_id: parent.id,
+                            max_size_kb: child.max_size_kb,
+                            is_active: true,
+                        })
+                        groupsByName.set(child.name, createdChild)
+                        createdGroups.push(createdChild)
+                    }
+
+                    const groupKey = String(createdChild.id)
+                    const existingSet = extensionsByGroup.get(groupKey) || new Set()
 
                     for (const ext of child.extensions || []) {
+                        const key = String(ext.ext || '').toLowerCase()
+                        if (existingSet.has(key)) {
+                            continue
+                        }
                         const createdExt = await createAttachmentExtension({
                             extension: ext.ext,
                             attachment_group_id: createdChild.id,
                             allowed_mimes: ext.mimes,
                         })
                         createdExtensions.push(createdExt)
+                        existingSet.add(key)
                     }
+                    extensionsByGroup.set(groupKey, existingSet)
                 }
             }
 
@@ -2677,6 +2706,16 @@ export default function Acp({ isAdmin }) {
                             <div className="d-flex justify-content-between align-items-center mb-3">
                                 <h5 className="mb-0">{t('attachment.groups_title')}</h5>
                                 <div className="d-flex align-items-center gap-2">
+                                    <Button
+                                        type="button"
+                                        className="bb-accent-button"
+                                        onClick={handleSeedAttachmentDefaults}
+                                        disabled={attachmentSeedSaving}
+                                    >
+                                        {attachmentSeedSaving
+                                            ? t('attachment.seed_in_progress')
+                                            : t('attachment.seed_defaults')}
+                                    </Button>
                                     <Button
                                         type="button"
                                         variant="outline-secondary"
@@ -2722,6 +2761,16 @@ export default function Acp({ isAdmin }) {
                             {!attachmentGroupsLoading && attachmentGroups.length > 0
                                 && attachmentGroups.every((group) => !group.parent_id) && (
                                 <div className="bb-muted mb-3">
+                                    <Button
+                                        type="button"
+                                        className="bb-accent-button me-2"
+                                        onClick={handleSeedAttachmentDefaults}
+                                        disabled={attachmentSeedSaving}
+                                    >
+                                        {attachmentSeedSaving
+                                            ? t('attachment.seed_in_progress')
+                                            : t('attachment.seed_defaults')}
+                                    </Button>
                                     <Button
                                         type="button"
                                         variant="outline-secondary"
diff --git a/routes/api.php b/routes/api.php
index 4b1c1f6..51971c8 100644
--- a/routes/api.php
+++ b/routes/api.php
@@ -9,6 +9,7 @@ use App\Http\Controllers\I18nController;
 use App\Http\Controllers\PortalController;
 use App\Http\Controllers\PostController;
 use App\Http\Controllers\PostThankController;
+use App\Http\Controllers\PreviewController;
 use App\Http\Controllers\SettingController;
 use App\Http\Controllers\StatsController;
 use App\Http\Controllers\ThreadController;
@@ -96,3 +97,4 @@ Route::post('/posts', [PostController::class, 'store'])->middleware('auth:sanctu
 Route::post('/posts/{post}/thanks', [PostThankController::class, 'store'])->middleware('auth:sanctum');
 Route::delete('/posts/{post}/thanks', [PostThankController::class, 'destroy'])->middleware('auth:sanctum');
 Route::delete('/posts/{post}', [PostController::class, 'destroy'])->middleware('auth:sanctum');
+Route::post('/preview', [PreviewController::class, 'preview'])->middleware('auth:sanctum');