Web Form Controls Having Multiple Values
A distinction must be made between two technically different topics:
  • The web form containing multiple controls having the same name attribute.
  • The web form containing a single control, but which allows multiple values to be selected.

Multiple controls having the same name attribute

According to w3c standard HTML forms can have multiple controls sharing the same name attribute. See: https://www.w3.org/TR/html52/sec-forms.html
For example:
<input type="checkbox" name="topping" value="bacon" />
<input type="checkbox" name="topping" value="cheese" />
<input type="checkbox" name="topping" value="onion" />
OpenEndpoints will combine all values submitted into a single input-from-request parameter, separated by a multiple-value-separator. The default separator is a double pipe ||.
The above example would result in a parameter "topping" that has the following value:
<parameter name="topping" value="bacon||cheese||onion"/>
It is possible to define any other multiple-value-separator. See: Multiple parameters supplied with the same name.

HTML input types allowing multiple values

Some HTML form controls allow to submit multiple values for a single control, for example:
<select name="topping" multiple="true">
<option value="bacon">Bacon</option>
<option value="cheese">Cheese</option>
<option value="onion">Onion</option>
On selecting multiple values, this would be submitted by the user agent in the same way as having multiple controls with the same name attribute. The result will be the same as above in OpenEndpoints:
<parameter name="topping" value="bacon||cheese||onion"/>

Uploading multiple files

It is possible to upload several files at the same time:
<input type="file" name="foo" multiple="true" />