file uploading validation

[khoathanglong]

The upload form should show the conditions that the files should adhere to. The form should show:

• what mime types are expected for a given source type, once the source type is selected.
• what the maximum size of a file is.

The mime type comes from source types request, the maximum size should be configurable, e.g. via an environment variable, since it depends on the deployment of the system. The same maximum should be configured in nginx as in the frontend.

Edit: clarification

Original issue:

• At the moment source type accepts all type files. Maybe we should get file type (extension) along with source type?

• No file size limitation required. Should we have limitation on a single file size and display it in the UI? (and total files size when support multiple file upload)