Avatar caching problem using GridFS

[wtsarchive]

Steps to reproduce:

1. Log in as a user and go to My Account -> Change avatar
2. Upload an avatar from computer
3. Verify that the avatar was uploaded in a chat room
4. Get URL of the avatar, which looks like this: /avatars/filename.jpg
5. Make cURL request to that URL: curl -vv http://pathtochat.com/avatars/filename.jpg
6. Make note of Last-Modified header.
7. Repeat steps 1-6 with a different avatar file

When uploading the avatar for the second time, Last-Modified header should be updated as well, but that doesn't seem the case. Upon further investigation it seems that the uploadDate field in avatars.files collection contains the first upload date and doesn't change when uploading another avatar. This leads to caching issues with some browsers showing old avatar instead of new one, because the browser sends If-Modified-Since header and the server then responds with 304 Not Modified.

I was digging through code and it seems to me that the issue is within the GridFS itself, which doesn't update uploadDate when it overwrites files. A possible workaround would be remove the file before re-uploading it again, but I can't seem to find where exactly storing the avatar file happens in the code.

[wtsarchive]

I've created a pull request for this. Tested on my machine and it worked correctly.

[rodrigok]

Closed via #1547

[wtsarchive]

@rodrigok Well that was quick :). Thanks!