[FIX] ir_http: return a 404 if binary content cannot be decoded

Commit 5a9e1af6 has the unfortunate side-effect of crashing early
if for any reason the content cannot be decoded.
However, simply ignoring that the content cannot be decoded is no better idea:
some functions pipe the result to decoding functions that crash the same.
The resulting traceback pollutes the log with uninformative message such as:
binascii.Error: Incorrect padding 5 0.002 0.016

In case the content cannot be decoded (data corruption, or simply missing file)
we return a clean 404 instead, which is morally almost equivalent,
and is clean even from functions that depend on binary_content.

opw 2072586

closes odoo/odoo#38261

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>

odoo/addons/base/models/ir_http.py

@@ -380,7 +380,11 @@ class IrHttp(models.AbstractModel):
                 filename = "%s-%s-%s" % (record._name, record.id, field)
 
         if not mimetype:
-            mimetype = guess_mimetype(base64.b64decode(content), default=default_mimetype)
+            try:
+                decoded_content = base64.b64decode(content)
+            except base64.binascii.Error:  # if we could not decode it, no need to pass it down: it would crash elsewhere...
+                return (404, [], None)
+            mimetype = guess_mimetype(decoded_content, default=default_mimetype)
 
         # extension
         _, existing_extension = os.path.splitext(filename)