The FormData interface provides a way to easily construct a set of key/value pairs representing form fields and their values, which can then be easily sent using the XMLHttpRequest.send() method. It uses the same format a form would use if the encoding type were set to “multipart/form-data”.

Constructor
FormData()
Creates a new FormData object.
Methods
FormData.append()
Appends a new value onto an existing key inside a FormData object, or adds the key if it does not already exist.
FormData.delete()
Deletes a key/value pair from a FormData object.
FormData.entries()
Returns an iterator allowing to go through all key/value pairs contained in this object.
FormData.get()
Returns the first value associated with a given key from within a FormData object.
FormData.getAll()
Returns an array of all the values associated with a given key from within a FormData.
FormData.has()
Returns a boolean stating whether a FormData object contains a certain key.
FormData.keys()
Returns an iterator allowing to go through all keys of the key/value pairs contained in this object.
FormData.set()
Sets a new value for an existing key inside a FormData object, or adds the key/value if it does not already exist.
FormData.values()
Returns an iterator allowing to go through all values contained in this object.

https://developer.mozilla.org/en-US/docs/Web/API/FormData/FormData 

https://developer.mozilla.org/en-US/docs/Web/API/FormData

Get a form action/URL with jquery

Get the form, ask for the action attribute:

$('#myForm').attr('action');

How to Create Image and File Upload in PHP with jQuery AJAX

https://www.cloudways.com/blog/the-basics-of-file-upload-in-php/