diff --git a/internal/handlers/post_text.go b/internal/handlers/post_text.go
index de25b44..acd63da 100644
--- a/internal/handlers/post_text.go
+++ b/internal/handlers/post_text.go
@@ -23,6 +23,19 @@ type postLocalePayload struct {
 	Text string `json:"text"`
 }
 
+// requestLangCode is the normalized locale from ?lang= or Accept-Language (zh, en, ja, …).
+func requestLangCode(r *http.Request) string {
+	raw := strings.TrimSpace(r.URL.Query().Get("lang"))
+	if raw == "" {
+		raw = r.Header.Get("Accept-Language")
+	}
+	raw = strings.TrimSpace(strings.Split(raw, ",")[0])
+	if raw == "" {
+		return "zh"
+	}
+	return translateNormalizePostLang(raw)
+}
+
 func (t postTextI18n) pick(r *http.Request) string {
 	return pickLangField(r, t.TextZh, t.TextEn, t.TextJa, t.TextKo, t.TextVi, t.TextId, t.TextMs)
 }
diff --git a/internal/handlers/post_types.go b/internal/handlers/post_types.go
index 59ab519..ac56073 100644
--- a/internal/handlers/post_types.go
+++ b/internal/handlers/post_types.go
@@ -1,13 +1,18 @@
 package handlers
 
-import "strings"
+import (
+	"regexp"
+	"strings"
+)
 
-// Valid post_type values (Browse filter chips + admin selector).
+// Valid post_type values (Browse filter chips; inferred on admin save).
 var validPostTypes = map[string]bool{
 	"image": true, "video": true, "music": true, "ppt": true, "pdf": true,
 	"link": true, "text": true, "archive": true,
 }
 
+var postTextURLPattern = regexp.MustCompile(`https?://`)
+
 func normalizePostType(raw string) string {
 	s := strings.ToLower(strings.TrimSpace(raw))
 	switch s {
@@ -33,3 +38,84 @@ func normalizePostType(raw string) string {
 	}
 	return "text"
 }
+
+// inferPostTypeFromContent picks post_type from attachments and text (admin no longer sends postType).
+func inferPostTypeFromContent(text string, attachments []attachmentInput) string {
+	hasText := strings.TrimSpace(text) != ""
+	if len(attachments) == 0 {
+		if hasText && postTextURLPattern.MatchString(text) {
+			return "link"
+		}
+		return "text"
+	}
+
+	var image, video, music, ppt, pdf, archive bool
+	for _, a := range attachments {
+		img, vid, mus, p, pd, ar := attachmentPostTypeSignals(a)
+		image = image || img
+		video = video || vid
+		music = music || mus
+		ppt = ppt || p
+		pdf = pdf || pd
+		archive = archive || ar
+	}
+
+	// Specific file types first; image beats video when both exist (e.g. multi-photo posts).
+	switch {
+	case ppt:
+		return "ppt"
+	case pdf && !image && !video:
+		return "pdf"
+	case archive && !image && !video && !music && !ppt && !pdf:
+		return "archive"
+	case music && !image && !video:
+		return "music"
+	case image:
+		return "image"
+	case video:
+		return "video"
+	case pdf:
+		return "pdf"
+	case archive:
+		return "archive"
+	case music:
+		return "music"
+	}
+	if hasText && postTextURLPattern.MatchString(text) {
+		return "link"
+	}
+	return "text"
+}
+
+func attachmentPostTypeSignals(a attachmentInput) (image, video, music, ppt, pdf, archive bool) {
+	kind := strings.ToLower(strings.TrimSpace(a.Kind))
+	mime := strings.ToLower(strings.TrimSpace(a.Mime))
+	fn := strings.ToLower(strings.TrimSpace(a.Filename))
+	if kind == "" && mime != "" {
+		kind = classifyAttachmentKind(mime, fn)
+	}
+
+	if kind == "video" || strings.HasPrefix(mime, "video/") {
+		video = true
+	}
+	if kind == "image" || strings.HasPrefix(mime, "image/") {
+		image = true
+	}
+	if strings.HasPrefix(mime, "audio/") ||
+		strings.HasSuffix(fn, ".mp3") || strings.HasSuffix(fn, ".wav") ||
+		strings.HasSuffix(fn, ".m4a") || strings.HasSuffix(fn, ".flac") || strings.HasSuffix(fn, ".aac") {
+		music = true
+	}
+	if strings.Contains(mime, "pdf") || strings.HasSuffix(fn, ".pdf") {
+		pdf = true
+	}
+	if strings.Contains(mime, "presentation") ||
+		strings.HasSuffix(fn, ".ppt") || strings.HasSuffix(fn, ".pptx") {
+		ppt = true
+	}
+	if strings.HasSuffix(fn, ".zip") || strings.HasSuffix(fn, ".rar") ||
+		strings.HasSuffix(fn, ".7z") || strings.Contains(mime, "zip") {
+		archive = true
+	}
+	return
+}
diff --git a/internal/handlers/posts_common.go b/internal/handlers/posts_common.go
index b7b0f69..2d96fbf 100644
--- a/internal/handlers/posts_common.go
+++ b/internal/handlers/posts_common.go
@@ -31,7 +31,8 @@ type PostDTO struct {
 	PostType       string          `json:"postType"`
 	CategoryID     int             `json:"categoryId,omitempty"`
 	CategorySlug   string          `json:"categorySlug,omitempty"`
-	Language       string                       `json:"language"`
+	Language       string                       `json:"language"` // UI locale from ?lang= (matches text selection)
+	SourceLanguage string                       `json:"sourceLanguage,omitempty"` // DB source metadata (admin input language)
 	Text           string                       `json:"text,omitempty"`
 	Localizations  map[string]postLocalePayload `json:"localizations"`
 	Attachments    []AttachmentDTO              `json:"attachments"`
@@ -212,9 +213,9 @@ func translateNormalizePostLang(lang string) string {
 }
 
 func validateAdminPost(ap *adminPost) error {
-	ap.PostType = normalizePostType(ap.PostType)
-	if ap.PostType == "" || !validPostTypes[ap.PostType] {
-		return fmt.Errorf("postType required (image, video, music, ppt, pdf, link, text, archive)")
+	ap.PostType = inferPostTypeFromContent(ap.Text, ap.Attachments)
+	if !validPostTypes[ap.PostType] {
+		ap.PostType = "text"
 	}
 	if len(ap.Attachments) > maxPostAttachments {
 		return fmt.Errorf("too many attachments (max %d)", maxPostAttachments)
@@ -389,11 +390,11 @@ func scanPostRow(r *http.Request, row pgx.Row) (PostDTO, postTextI18n, error) {
 	var id uuid.UUID
 	var catID *int
 	var slug *string
-	var lang, postType string
+	var sourceLang, postType string
 	var pub, updated, created *time.Time
 	err := row.Scan(
 		&id, &texts.TextZh, &texts.TextEn, &texts.TextJa, &texts.TextKo, &texts.TextVi, &texts.TextId, &texts.TextMs,
-		&lang, &postType, &catID, &slug, &dto.IsRecommended, &pub, &updated, &created,
+		&sourceLang, &postType, &catID, &slug, &dto.IsRecommended, &pub, &updated, &created,
 	)
 	if err != nil {
 		return dto, texts, err
@@ -406,7 +407,8 @@ func scanPostRow(r *http.Request, row pgx.Row) (PostDTO, postTextI18n, error) {
 	if slug != nil {
 		dto.CategorySlug = *slug
 	}
-	dto.Language = lang
+	dto.SourceLanguage = sourceLang
+	dto.Language = requestLangCode(r)
 	dto.Text = texts.pick(r)
 	dto.Localizations = texts.toLocalizations()
 	if pub != nil {