I think the Content-Type header in the request is missing the boundary parameter, leading to the .disp method not seeing the « file » part

Here is an example :

POST /upload HTTP/1.1
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW

------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="username"

john_doe
------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="profile_picture"; filename="profile.jpg"
Content-Type: image/jpeg

[Binary data of the JPEG file]
------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="metadata"
Content-Type: application/json

{"age": 30, "location": "New York"}
------WebKitFormBoundary7MA4YWxkTrZu0gW--j

You can prevent classes extending %Persistent to have storage (an extent) by using the 'NoExtent' keyword.
Concrete classes will inherit properties and methods, and generate their own storage strategy block for them on compile.
 

Class AbstractPersistentClass Extends %Persistent [Abstract,NoExtent]
{
Property Name As %String; 

Property Department As %String; 

Property Title As %String;
} 

Class Customers Extends AbstractPersistentClass
{ ... } 

Class Suppliers Extends AbstractPersistentClass
{ ... }