2.2.26 [W3C-HTML52] Section 188.8.131.52. Multipart form data
C0044: Form fields, including filename fields, are encoded in UTF-8 and are not approximated
The specification states:
... Multipart form data ... The multipart/form-data encoding algorithm is as follows: ... 5. ... File names included in the generated multipart/form-data resource (as part of file fields) must use the character encoding selected above, though the precise name may be approximated if necessary (e.g. newlines could be removed from file names, quotes could be changed to "%22", and characters not expressible in the selected character encoding could be replaced by other characters). ...
Form fields, including filename fields, are encoded in UTF-8 and are not approximated.