I discovered a bug in the way that the "accepted_types" option of the file picker is handled. When passing in several different extensions and mime-types in the array file_get_grouptype() in lib/filelib.php returns an numerically indexed array of extensions. Those are passed in js_init_call(), which json_encodes it and includes in the page on output.

When the array has consecutive indices starting at 0 (e.g. array(0 => 'jpg', 1 => 'png', 2 => 'svg') it gets encoded as an Array in JavaScript (e.g. ['jpg', 'png', 'svg']) and when the filepicker code turns that into the accepted_types parameter for the ajax call to the repository it works great there is no error.

When the array has non-consecutive indices (e.g. array(0 => 'jpg', 1 => 'png', 9 => 'svg') it gets encoded as an JavaScript Object instead (e.g.

which in turn gets turned into the string "[object Object]" when the filepicker code trys to turn it into the accepted_types parameter for the ajax call to the repository.