diff --git a/pybotx/models/attachments.py b/pybotx/models/attachments.py
index 9882eca3..14cccce4 100644
--- a/pybotx/models/attachments.py
+++ b/pybotx/models/attachments.py
@@ -501,6 +501,12 @@ def encode_rfc2397(content: bytes, mimetype: str) -> str:
     return f"data:{mimetype};base64,{b64_content}"
 
 
+def get_mimetype_by_filename(filename: str) -> str:
+    extension = filename.rsplit(".", 1)[-1].lower()
+
+    return EXTENSIONS_TO_MIMETYPES.get(extension, DEFAULT_MIMETYPE)
+
+
 class BotXAPIAttachment(UnverifiedPayloadBaseModel):
     file_name: str
     data: str
@@ -512,10 +518,7 @@ def from_file_attachment(
     ) -> "BotXAPIAttachment":
         assert attachment.content is not None
 
-        mimetype = EXTENSIONS_TO_MIMETYPES.get(
-            attachment.filename.split(".")[-1],
-            DEFAULT_MIMETYPE,
-        )
+        mimetype = get_mimetype_by_filename(attachment.filename)
 
         return cls(
             file_name=attachment.filename,
diff --git a/tests/test_attachments.py b/tests/test_attachments.py
index a3a87b6d..f4066c80 100644
--- a/tests/test_attachments.py
+++ b/tests/test_attachments.py
@@ -21,10 +21,12 @@
     AttachmentVideo,
     AttachmentVoice,
     BotAPIAttachment,
+    BotXAPIAttachment,
     Contact,
     IncomingAttachment,
     Link,
     Location,
+    OutgoingAttachment,
     convert_api_attachment_to_domain,
 )
 
@@ -81,6 +83,21 @@ async def default_handler(message: IncomingMessage, bot: Bot) -> None:
     assert read_content == b"Hello, world!\n"
 
 
+async def test__botx_api_attachment__uppercase_file_extension_mimetype() -> None:
+    # - Arrange -
+    attachment = OutgoingAttachment(
+        content=b"Hello, world!",
+        filename="image.PNG",
+    )
+
+    # - Act -
+    api_attachment = BotXAPIAttachment.from_file_attachment(attachment)
+
+    # - Assert -
+    assert api_attachment.file_name == "image.PNG"
+    assert api_attachment.data == "data:image/png;base64,SGVsbG8sIHdvcmxkIQ=="
+
+
 API_AND_DOMAIN_NON_FILE_ATTACHMENTS = (
     (
         {