Modulo contatti in Html che invia allegato a pagina php

[ilterribile]

Benvenuto.
Non mostri il codice del form, che è la cosa interessante per la soluzione del problema.
Il campo di input file si chiama "allegato"? Se sì, controlla di non aver commesso errori di battitura.
Nel tag <form> hai messo anche l'attributo enctype="multipart/form-data"? Altrimenti i file non arrivano allo script PHP.

<form method="post" action="contattiAvvalimenti.php">
<p>Ragione Sociale<span class="asterisco">*</span><u></u><u></u></p>
        <div class="input-group">
          <input type="text" class="form-control" maxlength="50" name="mittente" required>
          </input></div>
        <p>Email<span class="asterisco">*</span></p>
        <div class="input-group">
          <input type="email" class="form-control" maxlength="50" name="Email" required>
          </input>
        </div>
        <p>Recapito Telefonico<span class="asterisco">*</span></p>
<div class="input-group">
  <input type="tel" class="form-control" maxlength="15" name="Telefono" required>
  </input>
</div>
 <p>Referente</p>
<div class="input-group">
  <input type="tel" class="form-control" maxlength="30" name="Referente" >
  </input>
</div>

        Partita Iva<span class="asterisco">*</span>
<div class="input-group">
<input type="text" class="form-control" maxlength="50" name="Piva" required>
          </input>
        </div>
            <legend>Dati appalto</legend>
            Ente appaltante<span class="asterisco">*</span>
<div class="input-group">
<input type="text" class="form-control" maxlength="50" name="Ente" required>
          </input>
        </div>
        CIG<span class="asterisco">*</span>
<div class="input-group">
<input type="text" class="form-control" maxlength="50" name="CIG" required>
          </input>
        </div>
        Oggetto Gare<span class="asterisco">*</span>
<div class="input-group">
<input type="text" class="form-control" maxlength="50" name="oggettogare" required>
          </input>
        </div>
        Scadenza gare<span class="asterisco">*</span>
<div class="input-group">
<input type="text" class="form-control" maxlength="50" name="scadenzagara" id="datepicker" required>
          </input>
        </div>
        <legend>Requisiti Richiesti</legend>
        Categorie<span class="asterisco">*</span>
<div class="input-group">
    <select class="form-control" name="ReqCategorie" required>
 <option value="" selected>  </option>
  <option>OG1</option>
  <option>OG2</option>
  <option>OG3</option>
  <option>OG4</option>
  <option>OG5</option>
    <option>OG6</option>
  <option>OG7</option>
  <option>OG8</option>
  <option>OG9</option>
  <option>OG10</option>
  <option>OG11</option>
  <option>OG12</option>
    <option>OG13</option>
  <option>OS1</option>
  <option>OS2</option>
    <option>OS3</option>
  <option>OS4</option>
  <option>OS5</option>
  <option>OS6</option>
  <option>OS7</option>
  <option>OS8</option>
  <option>OS9</option>
  <option>OS10</option>
  <option>OS11</option>
  <option>OS12</option>
  <option>OS13</option>
  <option>OS14</option>
  <option>OS15</option>
  <option>OS16</option>
  <option>OS17</option>
  <option>OS18</option>
  <option>OS19</option>
  <option>OS20</option>
  <option>OS21</option>
  <option>OS22</option>
  <option>OS23</option>
  <option>OS24</option>
  <option>OS25</option>
  <option>OS26</option>
  <option>OS27</option>
  <option>OS28</option>
  <option>OS29</option>
    <option>OS30</option>
      <option>OS31</option>
        <option>OS32</option>
                <option>OS33</option>
        <option>OS34</option>







</select>
        </div>
        Classifiche<span class="asterisco">*</span>
<div class="input-group">
    <select class="form-control" name="ReqClassifiche" required>
     <option value="" selected>  </option>
  <option>I</option>
  <option>II</option>
  <option>III</option>
  <option>III-bis</option>
  <option>IV</option>
   <option>IV-bis</option>
     <option>V</option>
       <option>VI</option>
         <option>VII</option>
           <option>VIII</option>





</select>
        </div>
        Ulteriore Categoria
<div class="input-group">
    <select class="form-control" name="UlterioreCategoria">
   <option value="" selected>  </option>
  <option>OG1</option>
  <option>OG2</option>
  <option>OG3</option>
  <option>OG4</option>
  <option>OG5</option>
    <option>OG6</option>
  <option>OG7</option>
  <option>OG8</option>
  <option>OG9</option>
  <option>OG10</option>
  <option>OG11</option>
  <option>OG12</option>
    <option>OG13</option>
  <option>OS1</option>
  <option>OS2</option>
    <option>OS3</option>
  <option>OS4</option>
  <option>OS5</option>
  <option>OS6</option>
  <option>OS7</option>
  <option>OS8</option>
  <option>OS9</option>
  <option>OS10</option>
  <option>OS11</option>
  <option>OS12</option>
  <option>OS13</option>
  <option>OS14</option>
  <option>OS15</option>
  <option>OS16</option>
  <option>OS17</option>
  <option>OS18</option>
  <option>OS19</option>
  <option>OS20</option>
  <option>OS21</option>
  <option>OS22</option>
  <option>OS23</option>
  <option>OS24</option>
  <option>OS25</option>
  <option>OS26</option>
  <option>OS27</option>
  <option>OS28</option>
  <option>OS29</option>
    <option>OS30</option>
      <option>OS31</option>
        <option>OS32</option>
                <option>OS33</option>
        <option>OS34</option>







</select>
        </div>
         Ulteriori Classifiche
<div class="input-group">
    <select class="form-control" name="UlterioriClassifiche">
     <option value="" selected>  </option>
  <option>I</option>
  <option>II</option>
  <option>III</option>
  <option>III-bis</option>
  <option>IV</option>
   <option>IV-bis</option>
     <option>V</option>
       <option>VI</option>
         <option>VII</option>
           <option>VIII</option>





</select>
        </div>
        
        <div class="input-group">        Allega file


                                <input type="File" name="allegato"  id="allegato" accept="application/pdf,application/msword,application/x-zip-compressed"  size="10000"></input>
        </div>

                <div class="input-group">        <p align="center"> Note:</p>

<textarea class="form-control" rows="3" maxlenght="2000" name="Messaggio"></textarea>
</input>
        </div>
          <strong>Ho letto ed accetto le condizioni sulla <a href="privacy.html"><u>Privacy</u></a></strong>
<div class="input-group">
                                        <div class="boxinvia">

                                                      <button type="submit" class="btn btn-default btn-lg">Invia</button></div>

        </div>
                            
        </form>

Ho aggiunto anche enctype="multipart/form-data" nel campo form ma niente l'allegato non arriva ma il messaggio si