In your example you are posting to a mailto link. Formdata needs to be posted to the server which process your request. This could be any kind of programming language e.g. php, python, perl, .net etc.
When your server page receives the post request it can process the "post data". Here you program what the server should do with the data you have send. This could be for example storing in a database, store on harddisk or send an email with the image attached etc.
Here is a simple example:
HTML e.g. index.html
action="action.php"
refers to the file on the server which handles your post
<form action="action.php" method="post" enctype="multipart/form-data">
<input name="uploadedfile" type="file" />
<input type="submit" />
</form>
PHP e.g. action.php
The $_FILES
array is where PHP stores all the information about the files of your form.
move_uploaded_file($_FILES['uploadedfile']['tmp_name'], '/upload_files/myimage.jpg');
This moves the posted file to /upload_files/myimage.jpg on the server harddsik
see http://php.net/manual/en/function.move-uploaded-file.php