Allow caching of file endpoints

[irfinnew]

We now disabled caching of everything /api/ (see T6850), but that includes things like /api/logo/5/image/. This may negatively impact performance.

Ideally, we'd use aggressive caching for such things, and change the URL when the image changes. Maybe /api/logo/5/image/X/, where X is an incrementing serial, a hash/checksum, a timestamp, whatever.

[SpaceK33z]

The caching of images is essential if you load a couple of them in one page, e.g. for REX the performance without caching was disastrous on the dashboard (we have many contact avatars).

What we did there was something like this: api/contact/2/image/3/, where the 3 was the ID of the file. In the contact API it would have a image_url property, which would look like "image_url": "api/contact/2/image/3/".

[irfinnew]

Part of the problem is that under our current setup nginx is told to add no-cache headers to everything under /api/ ;)