FieldName is a string property that represents the name to associate with values stored in the form data field. FieldName is equivalent to the HTML ID or NAME attributes that identify an input field found in an HTML form. 

Changing the value in FieldName causes the value in FieldSize to be calculated and updated. 

FieldName is used when the multipart form data stream generates the formatted values (including headers) for use in an HTTP GET or POST request. 

Use FieldValue to specify the content to be associated with the named form data field.

TIdFormDataField.FieldSize 

TIdFormDataField.FieldValue