Attachment saved in variable with wp_mail() not being sent - php

Question

After having read the documentation for wp_mail several times and looking around for solutions, i still haven't been able to figure out how to get a file attached with wp_mail();
First of all, i'm of the understanding that the file path is needed, but how do i know where it is saved on my wordpress site? And when it is is saved, is it temporary or permanently?
Currently i'm just using $attachments = array(WP_PLUGIN_DIR . '/uploads/file_to_attach.zip');
because i saw it in another post on stackoverflow. But this means that i'm not using my $_POST['file']
in which the path should be stored.
HTML
<form method="post" role="form" action=''>
<div class="col-xs-12 col-sm-6 form-group">
<label>Navn</label>
<input class="form-control" name="name" type="text" value="<?php echo $_POST['name']; ?>" required/>
</div>
<div class="col-xs-12 col-sm-6 form-group">
<label>E-mail</label>
<input class="form-control" name="email" type="email" value="<?php echo $_POST['email']; ?>" required/>
</div>
<div class="col-xs-12 col-sm-6 form-group">
<label>Tlf.</label>
<input class="form-control" name="phone" type="tel" value="<?php echo $_POST['phone']; ?>" required/>
</div>
<div class="col-xs-12 col-sm-6 form-group">
<label>Firma</label>
<input class="form-control" name="company" type="text" value="<?php echo $_POST['company']; ?>" />
</div>
<div class="col-xs-12 form-group">
<label>Besked</label>
<textarea class="form-control" name="message" type="text" rows="6" required><?php echo $_POST['message']; ?></textarea>
</div>
<div class="col-xs-12 form-group">
<div class="input-group">
<label class="input-group-btn">
<span class="btn btn-primary">
Vedhæft billeder… <input type="file" name="file" style="display: none;" multiple>
</span>
</label>
<input type="text" class="form-control" readonly>
</div>
</div>
<div class="col-xs-12" id="checkrow">
<label>
<input type="checkbox" name="check" required> Jeg er et menneske
</label>
</div>
<div class="col-xs-4 form-group" style="padding-top:1rem;">
<input id="submit" name="submit" type="submit" value="Send" class="btn btn-primary" style="min-width:5rem">
</div>
<?php echo $result; ?>
</form>
PHP
$to = "ch#ap-pe.dk";
$msg = "Navn: ".$_POST['name']."
Email: ".$_POST['email']."
Tlf: ".$_POST['phone']."
Firma: ".$_POST['company']."
Besked: ".$_POST['message'];
$msg = wordwrap($msg, 70);
$headers = 'From: ';
$attachments = array(WP_PLUGIN_DIR . '/uploads/file_to_attach.zip');
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
if (!$_POST['name']){
$errorMsg="Indtast navn";
}
if (!$_POST['email']){
$errorMsg.="Indtast email";
}
if (!$_POST['message']){
$errorMsg.="Indtast besked";
}
if (!$_POST['check']){
$errorMsg.="Bekræft at du er menneske";
}
$result = "Ret følgende fejl: $errorMsg";
wp_mail($to, "Kontakt Besked",$msg, $headers, $attachments);
}
?>