For an <input type="file">, there are many scenarios that would result in an invalid file chosen, whether the file type is incorrect or the file size is too large, or there was an issue with the file ...

Iv'e double checked the the file's mime type is corect and Iv'e tryed with other .doc files, and I get the same result. Other allowed file types do stil work, so it seems to only be an issue with ...