Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/model/enclosure.go

@@ -0,0 +1,34 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package model // import "miniflux.app/v2/internal/model"
+import "strings"
+
+// Enclosure represents an attachment.
+type Enclosure struct {
+	ID               int64  `json:"id"`
+	UserID           int64  `json:"user_id"`
+	EntryID          int64  `json:"entry_id"`
+	URL              string `json:"url"`
+	MimeType         string `json:"mime_type"`
+	Size             int64  `json:"size"`
+	MediaProgression int64  `json:"media_progression"`
+}
+
+// Html5MimeType will modify the actual MimeType to allow direct playback from HTML5 player for some kind of MimeType
+func (e Enclosure) Html5MimeType() string {
+	if strings.HasPrefix(e.MimeType, "video") {
+		switch e.MimeType {
+		// Solution from this stackoverflow discussion:
+		// https://stackoverflow.com/questions/15277147/m4v-mimetype-video-mp4-or-video-m4v/66945470#66945470
+		// tested at the time of this commit (06/2023) on latest Firefox & Vivaldi on this feed
+		// https://www.florenceporcel.com/podcast/lfhdu.xml
+		case "video/m4v":
+			return "video/x-m4v"
+		}
+	}
+	return e.MimeType
+}
+
+// EnclosureList represents a list of attachments.
+type EnclosureList []*Enclosure