diff --git a/chatmastermind/commands/translation.py b/chatmastermind/commands/translation.py
index 9ed2945..938e8ec 100644
--- a/chatmastermind/commands/translation.py
+++ b/chatmastermind/commands/translation.py
@@ -1,4 +1,5 @@
 import argparse
+import mimetypes
 from pathlib import Path
 from .common import invert_input_tag_args, read_text_file
 from ..configuration import Config
@@ -13,6 +14,12 @@ class TranslationCmdError(Exception):
 text_separator: str = 'TEXT:'
 
 
+def assert_document_type_supported_openai(document_file: Path) -> None:
+    doctype = mimetypes.guess_type(document_file)
+    if doctype != 'text/plain':
+        raise TranslationCmdError("AI 'OpenAI' only supports document type 'text/plain''")
+
+
 def translation_prompt_openai(source_lang: str, target_lang: str) -> str:
     """
     Return the prompt for GPT that tells it to do the translation.
@@ -51,7 +58,8 @@ def create_message_openai(chat: ChatDB, args: argparse.Namespace) -> Message:
     user_text: str
     user_prompt: str
     if args.input_document is not None:
-        user_text = read_text_file(args.input_document)
+        assert_document_type_supported_openai(Path(args.input_document))
+        user_text = read_text_file(Path(args.input_document))
         user_prompt = '\n\n'.join([str(s) for s in text_args])
     else:
         user_text = text_args[-1]