diff --git a/src/app/file/ase_format.cpp b/src/app/file/ase_format.cpp
index 68ee01dc7..2e5b5e508 100644
--- a/src/app/file/ase_format.cpp
+++ b/src/app/file/ase_format.cpp
@@ -1338,13 +1338,13 @@ static void ase_file_write_external_files_chunk(
     return;
 
   ChunkWriter chunk(f, frame_header, ASE_FILE_CHUNK_EXTERNAL_FILE);
-  fputl(ext_files.to_fn.size(), f);        // Number of entries
+  fputl(ext_files.items.size(), f);         // Number of entries
   ase_file_write_padding(f, 8);
-  for (auto item : ext_files.to_fn) {
-    fputl(item.first, f);                  // ID
-    fputc(ext_files.types[item.first], f); // Type
+  for (const auto& it : ext_files.items) {
+    fputl(it.first, f);                     // ID
+    fputc(it.second.type, f);               // Type
     ase_file_write_padding(f, 7);
-    ase_file_write_string(f, item.second); // Filename
+    ase_file_write_string(f, it.second.fn); // Filename
   }
 }
 
diff --git a/src/dio/aseprite_common.h b/src/dio/aseprite_common.h
index 33afd4d6c..10ccdb1b9 100644
--- a/src/dio/aseprite_common.h
+++ b/src/dio/aseprite_common.h
@@ -106,25 +106,24 @@ struct AsepriteChunk {
 };
 
 struct AsepriteExternalFiles {
-  std::map<uint32_t, std::string> to_fn; // ID -> filename
+  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
-  std::map<uint32_t, uint8_t> types; // ID -> type (type has one of the ASE_EXTERNAL_FILE_* values)
 
   uint32_t lastid = 0;
 
   // Adds the external filename with the next autogenerated ID and specified type.
   void put(const std::string& filename, uint8_t type) {
-    auto id = ++lastid;
-    to_fn[id] = filename;
-    to_id[filename] = id;
-    types[id] = type;
+    put(++lastid, filename, type);
   }
 
   // Adds the external filename using the specified ID and type.
   void put(uint32_t id, const std::string& filename, uint8_t type) {
-    to_fn[id] = filename;
+    items[id] = Item{ filename, type };
     to_id[filename] = id;
-    types[id] = type;
   }
 
   // Returns true if the given filename exists in the external files
@@ -138,10 +137,10 @@ struct AsepriteExternalFiles {
   }
 
   bool getFilenameByID(uint32_t id, std::string& fn) const {
-    auto it = to_fn.find(id);
-    if (it == to_fn.end())
+    auto it = items.find(id);
+    if (it == items.end())
       return false;
-    fn = it->second;
+    fn = it->second.fn;
     return true;
   }
 };
diff --git a/src/dio/aseprite_decoder.cpp b/src/dio/aseprite_decoder.cpp
index e22edff81..aaeb8d5b2 100644
--- a/src/dio/aseprite_decoder.cpp
+++ b/src/dio/aseprite_decoder.cpp
@@ -1197,9 +1197,8 @@ doc::Tileset* AsepriteDecoder::readTilesetChunk(doc::Sprite* sprite,
     const uint32_t extFileId = read32(); // filename ID in the external files chunk
     const doc::tileset_index extTilesetId = read32(); // tileset ID in the external file
 
-    auto it = extFiles.to_fn.find(extFileId);
-    if (it != extFiles.to_fn.end()) {
-      auto fn = it->second;
+    std::string fn;
+    if (extFiles.getFilenameByID(extFileId, fn)) {
       tileset->setExternal(fn, extTilesetId);
     }
     else {