Send files in request using JSONAPI - ruby-on-rails

Question

I am using Rails 5 as the backend of a mobile app. The problem I am trying to solve is receiving a request from the app containing information about the customer plus 2 photos. After a short approach two options appeared:
Send the files first in a multipart/form-data POST, and return an ID to the client. After that a "real" request is sent again and the server should associate the ID(metadata) and the file.
Send the files is Base64.encoded format without changing the JSON header. Something like:
curl -X POST \
-H "Content-Type: application/vnd.api+json" \
-H "Cache-Control: no-cache" \
-d '{
"data": {
"type": "identities",
"attributes": {
"param1": "first param",
"param2": "second param",
"image1": "data:image/png;base64,iVBORw0KGgoAAAANSU.....",
"image2": "data:image/png;base64,iVBORw0KGgoAAAANSU....."
}
}' "http://API_URL/identity"
My concerns about these 2 approaches respectively are:
Since we are expecting 2 files should we make a request for each one associating it with ID? What is expected to happen if the second call doesn't reach the server or is not valid?
What amount of bytes should we accept? I was thinking of 10MB but I am not sure if this is a good idea and how the server will react? Is it a good idea firstly to validate the type and size of the file on a UI level(the mobile app)?
If someone can suggest something else I would really appreciate it. Also if you have any experience with this problem please share useful references that you have used, I would appreciate them as well.

Answer 1

1) If you need 2 files at one request - pass 2 files as multipart/form-data, that is totally fine. If you use b64, you encode everything first and then decode everything. Not the best idea.
2) You should validate those files both at front and back end. Max amount of bytes should be like max_file1_size + max_file_2_size + max_other_fields_size + headers_size, it's less about guessing then trying.
3) It will be a good option to use carrierwave - nice gem, and you'll got a lot less space to mess :)