diff --git a/CHANGES b/CHANGES
index dd035da26..f09dde19f 100644
--- a/CHANGES
+++ b/CHANGES
@@ -129,6 +129,7 @@ Bugs fixed
 * #6378: linkcheck: Send commonly used User-Agent
 * #6387: html search: failed to search document with haiku and scrolls themes
 * #6406: Wrong year is returned for ``SOURCE_DATE_EPOCH``
+* #6402: image directive crashes by unknown image format
 
 Testing
 --------
diff --git a/sphinx/environment/collectors/asset.py b/sphinx/environment/collectors/asset.py
index a0c009463..572dea7a4 100644
--- a/sphinx/environment/collectors/asset.py
+++ b/sphinx/environment/collectors/asset.py
@@ -100,6 +100,9 @@ class ImageCollector(EnvironmentCollector):
                                         filename)
             try:
                 mimetype = guess_mimetype(filename)
+                if mimetype is None:
+                    basename, suffix = path.splitext(filename)
+                    mimetype = 'image/x-' + suffix[1:]
                 if mimetype not in candidates:
                     globbed.setdefault(mimetype, []).append(new_imgpath)
             except OSError as err: