Impacted versions:
10.0

Steps to reproduce:

1.fresh install ubuntu 16.04.1 and odoo 10.0 latest source code and depends libs.
2.start odoo by run 
`/home/odoo/odoo-dev/odoo-server/odoo-bin -c /home/odoo/odoo-dev/community.conf`
3.open web browser, http://localhost:8069
enter new database options
- Database Name: demo, 
- Email: admin
- Password: as you want
- Language: English
- Country: United States (This is very important otherwise you cannot re-produce the issue)
- Check "Load demo data" checkbox
  click continue button

Current behavior:
Command line reported error during generating new database:

```
2016-10-14 06:32:46,817 2361 INFO demo odoo.addons.base.ir.ir_http: Generating routing map
2016-10-14 06:32:52,291 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/ HTTP/1.1" 200 -
2016-10-14 06:32:52,336 2361 INFO demo odoo.addons.base.ir.ir_attachment: filestore gc 349 checked, 9 removed
2016-10-14 06:32:52,357 2361 INFO demo odoo.addons.base.ir.ir_attachment: _read_file reading /home/odoo/.local/share/Odoo/filestore/demo/2e/2ef882f84c89c3634d210d011ec61b9144cddc13
Traceback (most recent call last):
  File "/home/odoo/odoo-dev/odoo-server/odoo/addons/base/ir/ir_attachment.py", line 100, in _file_read
    r = open(full_path,'rb').read().encode('base64')
IOError: [Errno 2] No such file or directory: u'/home/odoo/.local/share/Odoo/filestore/demo/2e/2ef882f84c89c3634d210d011ec61b9144cddc13'
2016-10-14 06:32:52,383 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/content/342-825761c/web.assets_common.0.css HTTP/1.1" 200 -
2016-10-14 06:32:52,401 2361 INFO demo odoo.addons.base.ir.ir_autovacuum: GC'd 0 user log entries
2016-10-14 06:32:52,416 2361 INFO demo odoo.addons.base.ir.ir_attachment: _read_file reading /home/odoo/.local/share/Odoo/filestore/demo/ef/ef90a0c13bc75a6927d4dc43a6997051168f2287
Traceback (most recent call last):
  File "/home/odoo/odoo-dev/odoo-server/odoo/addons/base/ir/ir_attachment.py", line 100, in _file_read
    r = open(full_path,'rb').read().encode('base64')
IOError: [Errno 2] No such file or directory: u'/home/odoo/.local/share/Odoo/filestore/demo/ef/ef90a0c13bc75a6927d4dc43a6997051168f2287'
2016-10-14 06:32:52,419 2361 INFO demo odoo.addons.base.ir.ir_attachment: _read_file reading /home/odoo/.local/share/Odoo/filestore/demo/51/514b7968142c494c569d209d184985ae5babac45
Traceback (most recent call last):
  File "/home/odoo/odoo-dev/odoo-server/odoo/addons/base/ir/ir_attachment.py", line 100, in _file_read
    r = open(full_path,'rb').read().encode('base64')
IOError: [Errno 2] No such file or directory: u'/home/odoo/.local/share/Odoo/filestore/demo/51/514b7968142c494c569d209d184985ae5babac45'
2016-10-14 06:32:52,436 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/content/343-a7d549d/web.assets_backend.0.css HTTP/1.1" 200 -
2016-10-14 06:32:52,440 2361 INFO demo odoo.addons.base.ir.ir_attachment: _read_file reading /home/odoo/.local/share/Odoo/filestore/demo/37/37e8b0675e0d2722861b1bbbf31d1ca8f0816605
Traceback (most recent call last):
  File "/home/odoo/odoo-dev/odoo-server/odoo/addons/base/ir/ir_attachment.py", line 100, in _file_read
    r = open(full_path,'rb').read().encode('base64')
IOError: [Errno 2] No such file or directory: u'/home/odoo/.local/share/Odoo/filestore/demo/37/37e8b0675e0d2722861b1bbbf31d1ca8f0816605'
2016-10-14 06:32:52,447 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/content/344-a7d549d/web.assets_backend.1.css HTTP/1.1" 200 -
2016-10-14 06:32:52,447 2361 INFO demo odoo.addons.base.ir.ir_attachment: _read_file reading /home/odoo/.local/share/Odoo/filestore/demo/cf/cf7435f9c0959a8ff500c5d8b1e82de480ca731d
Traceback (most recent call last):
  File "/home/odoo/odoo-dev/odoo-server/odoo/addons/base/ir/ir_attachment.py", line 100, in _file_read
    r = open(full_path,'rb').read().encode('base64')
IOError: [Errno 2] No such file or directory: u'/home/odoo/.local/share/Odoo/filestore/demo/cf/cf7435f9c0959a8ff500c5d8b1e82de480ca731d'
2016-10-14 06:32:52,468 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/content/347-825761c/web.assets_common.js HTTP/1.1" 200 -
2016-10-14 06:32:52,477 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/content/345-ce848ff/web_editor.summernote.0.css HTTP/1.1" 200 -
2016-10-14 06:32:52,479 2361 INFO demo odoo.addons.base.ir.ir_attachment: _read_file reading /home/odoo/.local/share/Odoo/filestore/demo/b3/b34488fd73a751b68cc014731e625e10130f8d02
Traceback (most recent call last):
  File "/home/odoo/odoo-dev/odoo-server/odoo/addons/base/ir/ir_attachment.py", line 100, in _file_read
    r = open(full_path,'rb').read().encode('base64')
IOError: [Errno 2] No such file or directory: u'/home/odoo/.local/share/Odoo/filestore/demo/b3/b34488fd73a751b68cc014731e625e10130f8d02'
2016-10-14 06:32:52,497 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/content/346-cbe0fa6/web_editor.assets_editor.0.css HTTP/1.1" 200 -
2016-10-14 06:32:52,497 2361 INFO demo odoo.addons.base.ir.ir_attachment: _read_file reading /home/odoo/.local/share/Odoo/filestore/demo/e1/e1bf7b2e69460e68f9e6fb61fece8fa83f7bc96b
Traceback (most recent call last):
  File "/home/odoo/odoo-dev/odoo-server/odoo/addons/base/ir/ir_attachment.py", line 100, in _file_read
    r = open(full_path,'rb').read().encode('base64')
IOError: [Errno 2] No such file or directory: u'/home/odoo/.local/share/Odoo/filestore/demo/e1/e1bf7b2e69460e68f9e6fb61fece8fa83f7bc96b'
2016-10-14 06:32:52,498 2361 INFO demo odoo.addons.base.ir.ir_attachment: _read_file reading /home/odoo/.local/share/Odoo/filestore/demo/38/38b74389d6dfab413e09aedfb39028afdada5725
Traceback (most recent call last):
  File "/home/odoo/odoo-dev/odoo-server/odoo/addons/base/ir/ir_attachment.py", line 100, in _file_read
    r = open(full_path,'rb').read().encode('base64')
IOError: [Errno 2] No such file or directory: u'/home/odoo/.local/share/Odoo/filestore/demo/38/38b74389d6dfab413e09aedfb39028afdada5725'
2016-10-14 06:32:52,503 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/content/349-ce848ff/web_editor.summernote.js HTTP/1.1" 200 -
2016-10-14 06:32:52,505 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/content/348-a7d549d/web.assets_backend.js HTTP/1.1" 200 -
2016-10-14 06:32:52,508 2361 INFO demo odoo.addons.base.ir.ir_attachment: _read_file reading /home/odoo/.local/share/Odoo/filestore/demo/77/77ba38750ee153d70e633e7fd93aa60af588fe76
Traceback (most recent call last):
  File "/home/odoo/odoo-dev/odoo-server/odoo/addons/base/ir/ir_attachment.py", line 100, in _file_read
    r = open(full_path,'rb').read().encode('base64')
IOError: [Errno 2] No such file or directory: u'/home/odoo/.local/share/Odoo/filestore/demo/77/77ba38750ee153d70e633e7fd93aa60af588fe76'
2016-10-14 06:32:52,509 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/content/350-cbe0fa6/web_editor.assets_editor.js HTTP/1.1" 200 -
2016-10-14 06:32:52,518 2361 INFO demo werkzeug: 127.0.0.1 - - [14/Oct/2016 06:32:52] "GET /web/binary/company_logo HTTP/1.1" 200 -
```

<img width="455" alt="reportbug1" src="https://cloud.githubusercontent.com/assets/1243299/19380336/7157f282-9229-11e6-8ecc-4067a7e210b6.png">
The web page is abnormal.
After checking the code, I found the issue may cause by -- odoo.addons.base.ir.ir_attachment: filestore gc.
The "_file_gc" function in ir_attachment.py deleted these useful 9 files, such as filestore/demo/2e/2ef882f84c89c3634d210d011ec61b9144cddc13 ......
Note: this bug only happened in the first time we create new database with above options in a new install environment. 
I guess maybe "_file_gc" does not get correct parameters during the process.
It be invoked too early, and these 9 files has not been added in gc checklist yet.

Expected behavior:
The "_file_gc" function does not deleted these useful 9 files.
Video/Screenshot link (optional):

return
        
        cr = self._cr
        cr.execute("LOCK ir_attachment IN SHARE MODE")
        
        checklist = {}
        for dirpath, _, filenames in os.walk(self._full_path('checklist')):
            dirname = os.path.basename(dirpath)
            for filename in filenames:
                fname = "%s/%s" % (dirname, filename)
                checklist[fname] = os.path.join(dirpath, filename)
        
        whitelist = set()
        for names in cr.split_for_in_conditions(checklist):
            cr.execute("SELECT store_fname FROM ir_attachment WHERE store_fname IN %s", [names])
            whitelist.update(row[0] for row in cr.fetchall())
        
        removed = 0
        for fname, filepath in checklist.iteritems():
            if fname not in whitelist:
                try:
                    os.unlink(self._full_path(fname))
                    removed += 1
                except (OSError, IOError):
                    _logger.info("_file_gc could not unlink %s", self._full_path(fname), exc_info=True)
            with tools.ignore(OSError):
                os.unlink(filepath)
        _logger.info("filestore gc %d checked, %d removed", len(checklist), removed)
    @api.depends('store_fname', 'db_datas')
    def _compute_datas(self):