Content_type and content_size of file before save it

[Alex jimmy Montaño fuentes]

<%= form_tag('URL', :multipart => true) do %>
<p>
Video: <%= file_field_tag :avatar %>
</p>
<p>
<%= submit_tag %>
</p>
<% end %>

Is there any way to know the content_type and content_size of file
before submit the form and send them as params to the POST request?

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