Can customizable products work out price based on the number of uploaded files?
The answer is "yes"

I originally posted the answer here: https://drupalcommerce.org/questions/16291/can-customizable-products-work-out-price-based-number-uploaded-files

Export of the rule that proves it's possible:

   { "rules_files_price_multiplier" : {
      "LABEL" : "Files Price Multiplier",
      "PLUGIN" : "reaction rule",
      "OWNER" : "rules",
      "REQUIRES" : [ "rules", "commerce_line_item", "commerce_product_reference" ],
      "ON" : { "commerce_product_calculate_sell_price" : [] },
      "IF" : [
        { "entity_is_of_type" : { "entity" : [ "commerce-line-item" ], "type" : "commerce_line_item" } },
        { "entity_has_field" : {
            "entity" : [ "commerce-line-item" ],
            "field" : "field_upload_all_the_files"
          }
        },
        { "list_count_is" : {
            "list" : [ "commerce-line-item:field-upload-all-the-files" ],
            "op" : "\u003E",
            "value" : "0"
          }
        }
      ],
      "DO" : [
        { "variable_add" : {
            "USING" : { "type" : "integer", "value" : "1" },
            "PROVIDE" : { "variable_added" : { "file_multiplier" : "file_multiplier" } }
          }
        },
        { "LOOP" : {
            "USING" : { "list" : [ "commerce-line-item:field-upload-all-the-files" ] },
            "ITEM" : { "list_item" : "Current list item" },
            "DO" : [
              { "data_calc" : {
                  "USING" : { "input_1" : [ "file-multiplier" ], "op" : "+", "input_2" : "1" },
                  "PROVIDE" : { "result" : { "result" : "Calculation result" } }
                }
              },
              { "data_set" : { "data" : [ "file-multiplier" ], "value" : [ "result" ] } }
            ]
          }
        },
        { "commerce_line_item_unit_price_multiply" : {
            "commerce_line_item" : [ "commerce_line_item" ],
            "amount" : [ "file-multiplier" ],
            "component_name" : "fee",
            "round_mode" : "1"
          }
        },
        { "drupal_message" : { "message" : "[commerce-line-item:line-item-id]: File Multiplier = [file-multiplier:value]" } }
      ]
    }
  }


NOTE: This will still not update the price as the person is uploading files.