diff --git a/news/251-extension-str-replace b/news/251-extension-str-replace
new file mode 100644
index 00000000..294e4eb7
--- /dev/null
+++ b/news/251-extension-str-replace
@@ -0,0 +1,20 @@
+### Enhancements
+
+* <news item>
+
+### Bug fixes
+
+* Replace `.conda` or `.tar.bz2` extensions from end of string only instead of
+  `str.replace(...)` (#251)
+
+### Deprecations
+
+* <news item>
+
+### Docs
+
+* <news item>
+
+### Other
+
+* <news item>
diff --git a/news/improve-typing b/news/improve-typing
new file mode 100644
index 00000000..0e177fed
--- /dev/null
+++ b/news/improve-typing
@@ -0,0 +1,19 @@
+### Enhancements
+
+* <news item>
+
+### Bug fixes
+
+* <news item>
+
+### Deprecations
+
+* <news item>
+
+### Docs
+
+* <news item>
+
+### Other
+
+* Improve type annotations on an internal function (#257)
diff --git a/src/conda_package_handling/api.py b/src/conda_package_handling/api.py
index c8d572f3..47881ce4 100644
--- a/src/conda_package_handling/api.py
+++ b/src/conda_package_handling/api.py
@@ -204,10 +204,11 @@ def transmute(in_file, out_ext, out_folder=None, processes=1, **kw):
         out_folder = _os.path.dirname(in_file) or _os.getcwd()
 
     flist = set(_glob(in_file))
-    if in_file.endswith(".tar.bz2"):
-        flist = flist - set(_glob(in_file.replace(".tar.bz2", out_ext)))
-    elif in_file.endswith(".conda"):
-        flist = flist - set(_glob(in_file.replace(".conda", out_ext)))
+    for in_ext in SUPPORTED_EXTENSIONS:
+        if in_file.endswith(in_ext):
+            replacement = in_file[: -len(in_ext)] + out_ext
+            flist = flist - set(_glob(replacement))
+            break
 
     failed_files = {}
     with _get_executor(processes) as executor:
diff --git a/src/conda_package_handling/conda_fmt.py b/src/conda_package_handling/conda_fmt.py
index 5c363b4b..5342d622 100644
--- a/src/conda_package_handling/conda_fmt.py
+++ b/src/conda_package_handling/conda_fmt.py
@@ -69,7 +69,7 @@ def create(
             out_folder = os.path.dirname(out_fn)
             out_fn = os.path.basename(out_fn)
         conda_pkg_fn = os.path.join(out_folder, out_fn)
-        file_id = out_fn.replace(".conda", "")
+        file_id = out_fn[: -len(".conda")]
         pkg_files = utils.filter_info_files(file_list, prefix)
         # preserve order
         pkg_files_set = set(pkg_files)
diff --git a/src/conda_package_handling/streaming.py b/src/conda_package_handling/streaming.py
index 4f367a43..f05f981a 100644
--- a/src/conda_package_handling/streaming.py
+++ b/src/conda_package_handling/streaming.py
@@ -5,9 +5,10 @@
 from __future__ import annotations
 
 import io
+import tarfile
 from contextlib import redirect_stdout
-from tarfile import TarError, TarFile, TarInfo
-from typing import Iterator
+from tarfile import TarError
+from typing import Generator
 from zipfile import BadZipFile
 
 from conda_package_streaming.extract import exceptions as cps_exceptions
@@ -20,7 +21,7 @@ def _stream_components(
     filename: str,
     components: list[str],
     dest_dir: str = "",
-) -> Iterator[tuple[TarFile, TarInfo]]:
+) -> Generator[Generator[tuple[tarfile.TarFile, tarfile.TarInfo]]]:
     if str(filename).endswith(".tar.bz2"):
         assert components == ["pkg"]