Rails - Sending image using Faraday - ruby-on-rails

Question

I'm trying to make a request to an API sending an image and some other data, and getting the response. That's my code:
file = "assets/images/test.jpg"
conn = Faraday.new(:url => "api_url" ) do |faraday|
faraday.request :multipart
end
payload = { :profile_pic => Faraday::UploadIO.new(file, 'image/jpeg') }
conn.post "/test", payload
My first problem is that I'm always getting the following error:
Errno::ENOENT (No such file or directory - assets/images/test.png)
I've tried all the paths I could imagine. Where should be saved the image in directories to be found by Faraday?
The second question is about the response, how can I get the response and handle it?
The third one is that, I haven't understand what's the utility of the first parameter of the last call:
conn.post "/hello", payload
I've written "/hello" but don't have any idea about what's the real usage.
And the last one. Could I send a raw image saved in a variable instead of sending a path to Faraday?
EDIT
Now it's working, this is the solution:
Be aware that url must be only until .com, the rest of the path must go on conn.post like this example /v1/search.
c.adapter :net_http was needed too.
Message response is correctly handled in json variable.
Solution:
url = 'http://url.com'
file = Rails.root.to_s + "/app/assets/images/test.jpg"
conn = Faraday.new(:url => url ) do |c|
c.request :multipart
c.adapter :net_http
end
payload = { :image => Faraday::UploadIO.new(file, 'image/jpeg'), :token => token}
response = conn.post '/v1/search', payload
json = JSON.parse response.body

Answer 1

You should try this for your first question :
file = Rails.root.to_s + "/app/assets/images/test.jpg"
For your third question, the first parameters allows you to construct the right URL from the base "api_url". Please see the example from the Readme.
## POST ##
conn.post '/nigiri', { :name => 'Maguro' } # POST "name=maguro" to http://sushi.com/nigiri