set_avatar_from_file: raise if file isn't an image

src/python/errors.py

@@ -62,6 +62,13 @@ class InvalidUserInContext(Exception):
 class UneededThumbnail(Exception):
     pass
 
+
 @dataclass
 class UnthumbnailableError(Exception):
     exception: Optional[Exception] = None
+
+
+@dataclass
+class BadMimeType(Exception):
+    wanted: str = field()
+    got:    str = field()

src/python/matrix_client.py

@@ -25,8 +25,8 @@ import nio
 
 from . import __about__, utils
 from .errors import (
-    InvalidUserInContext, MatrixError, UneededThumbnail, UnthumbnailableError,
-    UserNotFound,
+    BadMimeType, InvalidUserInContext, MatrixError, UneededThumbnail,
+    UnthumbnailableError, UserNotFound,
 )
 from .html_filter import HTML_FILTER
 from .models.items import (
@@ -617,7 +617,11 @@ class MatrixClient(nio.AsyncClient):
 
 
     async def set_avatar_from_file(self, path: Union[Path, str]) -> None:
-        # TODO: check if mime is image
+        mime = utils.guess_mime(path)
+
+        if mime.split("/")[0] != "image":
+            raise BadMimeType(wanted="image/*", got=mime)
+
         await self.set_avatar((await self.upload_file(path))[0])