diff --git a/src/app/file/ase_format.cpp b/src/app/file/ase_format.cpp
index 2e5b5e508..708fa8b27 100644
--- a/src/app/file/ase_format.cpp
+++ b/src/app/file/ase_format.cpp
@@ -1278,16 +1278,19 @@ static void ase_file_write_external_files_chunk(
   dio::AsepriteExternalFiles& ext_files,
   const Sprite* sprite)
 {
-  auto putExtentionIds = [](const UserData::PropertiesMaps& propertiesMaps, dio::AsepriteExternalFiles& ext_files) {
+  auto putExtentionIds = [](const UserData::PropertiesMaps& propertiesMaps,
+                            dio::AsepriteExternalFiles& ext_files) {
       for (auto propertiesMap : propertiesMaps) {
         if (!propertiesMap.first.empty())
-          ext_files.put(propertiesMap.first, ASE_EXTERNAL_FILE_EXTENSION);
+          ext_files.insert(ASE_EXTERNAL_FILE_EXTENSION,
+                           propertiesMap.first);
       }
   };
 
   for (const Tileset* tileset : *sprite->tilesets()) {
     if (!tileset->externalFilename().empty()) {
-      ext_files.put(tileset->externalFilename(), ASE_EXTERNAL_FILE_TILESET);
+      ext_files.insert(ASE_EXTERNAL_FILE_TILESET,
+                       tileset->externalFilename());
     }
 
     putExtentionIds(tileset->userData().propertiesMaps(), ext_files);
@@ -1334,13 +1337,13 @@ static void ase_file_write_external_files_chunk(
   }
 
   // No external files to write
-  if (ext_files.lastid == 0)
+  if (ext_files.items().empty())
     return;
 
   ChunkWriter chunk(f, frame_header, ASE_FILE_CHUNK_EXTERNAL_FILE);
-  fputl(ext_files.items.size(), f);         // Number of entries
+  fputl(ext_files.items().size(), f);       // Number of entries
   ase_file_write_padding(f, 8);
-  for (const auto& it : ext_files.items) {
+  for (const auto& it : ext_files.items()) {
     fputl(it.first, f);                     // ID
     fputc(it.second.type, f);               // Type
     ase_file_write_padding(f, 7);
@@ -1395,9 +1398,9 @@ static void ase_file_write_tileset_chunk(FILE* f, FileOp* fop,
 
   // Flag 1 = external tileset
   if (flags & ASE_TILESET_FLAG_EXTERNAL_FILE) {
-    auto it = ext_files.to_id.find(tileset->externalFilename());
-    if (it != ext_files.to_id.end()) {
-      auto file_id = it->second;
+    uint32_t file_id = 0;
+    if (ext_files.getIDByFilename(ASE_EXTERNAL_FILE_TILESET,
+                                  tileset->externalFilename(), file_id)) {
       fputl(file_id, f);
       fputl(tileset->externalTileset(), f);
     }
@@ -1532,7 +1535,8 @@ static void ase_file_write_properties_maps(FILE* f, FileOp* fop,
     const std::string& extensionKey = propertiesMap.first;
     uint32_t extensionId = 0;
     if (!extensionKey.empty() &&
-        !ext_files.getIDByFilename(extensionKey, extensionId)) {
+        !ext_files.getIDByFilename(ASE_EXTERNAL_FILE_EXTENSION,
+                                   extensionKey, extensionId)) {
       // This shouldn't ever happen, but if it does...  most likely
       // it is because we forgot to add the extensionID to the
       // ext_files object. And this could happen if someone adds the
diff --git a/src/dio/aseprite_common.h b/src/dio/aseprite_common.h
index 10ccdb1b9..c8d1847d4 100644
--- a/src/dio/aseprite_common.h
+++ b/src/dio/aseprite_common.h
@@ -66,6 +66,7 @@
 #define ASE_EXTERNAL_FILE_PALETTE           0
 #define ASE_EXTERNAL_FILE_TILESET           1
 #define ASE_EXTERNAL_FILE_EXTENSION         2
+#define ASE_EXTERNAL_FILE_TYPES             3
 
 namespace dio {
 
@@ -105,44 +106,67 @@ struct AsepriteChunk {
   int start;
 };
 
-struct AsepriteExternalFiles {
+class AsepriteExternalFiles {
+public:
   struct Item {
     std::string fn;             // filename
     uint8_t type;               // type has one of the ASE_EXTERNAL_FILE_* values
   };
-  std::map<uint32_t, Item> items;
-  std::map<std::string, uint32_t> to_id; // filename -> ID
 
-  uint32_t lastid = 0;
+  using Items = std::map<uint32_t, Item>;
+
+  const Items& items() const {
+    return m_items;
+  }
 
   // Adds the external filename with the next autogenerated ID and specified type.
-  void put(const std::string& filename, uint8_t type) {
-    put(++lastid, filename, type);
+  uint32_t insert(const uint8_t type,
+                  const std::string& filename) {
+    auto it = m_toID[type].find(filename);
+    if (it != m_toID[type].end())
+      return it->second;
+    else {
+      insert(++m_lastid, type, filename);
+      return m_lastid;
+    }
   }
 
   // Adds the external filename using the specified ID and type.
-  void put(uint32_t id, const std::string& filename, uint8_t type) {
-    items[id] = Item{ filename, type };
-    to_id[filename] = id;
+  void insert(uint32_t id,
+              const uint8_t type,
+              const std::string& filename) {
+    ASSERT(type >= 0 && type < ASE_EXTERNAL_FILE_TYPES);
+
+    m_items[id] = Item{ filename, type };
+    m_toID[type][filename] = id;
   }
 
   // Returns true if the given filename exists in the external files
   // chunk, and assign its ID in "id"
-  bool getIDByFilename(const std::string& fn, uint32_t& id) const {
-    auto it = to_id.find(fn);
-    if (it == to_id.end())
+  bool getIDByFilename(const uint8_t type,
+                       const std::string& fn,
+                       uint32_t& id) const {
+    ASSERT(type >= 0 && type < ASE_EXTERNAL_FILE_TYPES);
+
+    auto it = m_toID[type].find(fn);
+    if (it == m_toID[type].end())
       return false;
     id = it->second;
     return true;
   }
 
   bool getFilenameByID(uint32_t id, std::string& fn) const {
-    auto it = items.find(id);
-    if (it == items.end())
+    auto it = m_items.find(id);
+    if (it == m_items.end())
       return false;
     fn = it->second.fn;
     return true;
   }
+
+private:
+  uint32_t m_lastid = 0; // ID used to add new items
+  Items m_items;
+  std::map<std::string, uint32_t> m_toID[ASE_EXTERNAL_FILE_TYPES];
 };
 
 } // namespace dio
diff --git a/src/dio/aseprite_decoder.cpp b/src/dio/aseprite_decoder.cpp
index aaeb8d5b2..7d27c15d2 100644
--- a/src/dio/aseprite_decoder.cpp
+++ b/src/dio/aseprite_decoder.cpp
@@ -1014,7 +1014,7 @@ void AsepriteDecoder::readExternalFiles(AsepriteExternalFiles& extFiles)
     uint8_t type = read8();
     readPadding(7);
     std::string fn = readString();
-    extFiles.put(id, fn, type);
+    extFiles.insert(id, type, fn);
   }
 }