Your problem might better be solved by using variables to contain your selectors. By using a mixin, you’re losing the ability to chain it with similar elements.
$form-input-text: 'input[type="text"], input[type="password"], input[type="search"], input[type="email"], input[type="tel"], input[type="url"]';
$form-input-buttons: 'input[type="submit"], input[type="reset"], input[type="button"], button';
$form-input-dates: 'input[type^="date"], input[type="month"], input[type="week"], input[type="time"]';
$form-input-not-radio: 'input:not([type="radio"]):not([type="checkbox"])';
#{$form-input-text}, textarea {
@include border-radius(.25em);
border: $form-input-border;
}
#{$form-input-text}, textarea, input[type="file"] {
width: $form-input-width;
max-width: 100%;
-webkit-appearance: textfield
}
#{$form-input-buttons} {
padding: .25em .5em;
}