Fix XSD definition of a multivalued Blob property

Steps to reproduce:

1. define a document type with a schema named "file_schema" which has a multivalued Blob property
2. generate the code for this property in the View layout of this document

       <div role="widget">
         <label>Attachments</label>
         <nuxeo-document-attachments document="{{document}}" xpath="file_schema:attachments" name="attachments"></nuxeo-document-attachments>
       </div>
   

3. deploy the Studio project
4. create a document of the new type
5. on the summary, try to add a file to the zone for "file_schema:attachments"
6. observe that the file is not attached

Server side, "file_schema:attachments" is recognized as a BlobProperty (which cannot set multiple values) whereas "files:files" is recognized as a MapProperty

I've compared the definition generated by Studio with the default definition of "files:files"

  <xs:complexType name="blobList">
    <xs:sequence> 
      <xs:element name="item" type="nxs:content" minOccurs="0" maxOccurs="unbounded"/>
    </xs:sequence> 
  </xs:complexType>  
  
  <xs:element name="attachments" type="nxs:blobList"/>

whereas files:files is defined by

  <xs:element name="files" type="nxs:files" />

  <xs:complexType name="files">
    <xs:sequence>
      <xs:element name="item" type="nxs:file" minOccurs="0"
        maxOccurs="unbounded" />
    </xs:sequence>
  </xs:complexType>

  <xs:complexType name="file">
    <xs:sequence>
      <xs:element name="file" type="nxs:content" />
    </xs:sequence>
  </xs:complexType>