If you’re downloading a file with widely-recognized lossy compression, your user’s cellular provider may interfere with it. This has always been true of internet connections; I first ran into this with dialup years ago. But it went away for a while with broadband, is back with wireless.
The simplest example is a JPEG. You may get the JPEG you expect, but it’s also possible for the proxy to deliver a smaller JPEG than you expect. The cell provider considers it “close enough”, and the doubly-compressed JPEG is smaller (and far uglier).
This may be true of other file types as well, if they’re commonly recognized as lossy and computationally easy to re-compress.
I have read reports of T-Mobile and O2 doing this. I think it’s been noted with other cell providers, too.
There’s two possible fixes:
- Switching to HTTPS will fix this problem as the proxy will no longer be in the middle of the communication. If your resource is available via HTTPS, this is a fantastic and simple fix.
- Add a HTTP header to defeat this. The idea is to add a no-transform cache-control HTTP header to your request. This should disallow the server from making this kind of change. See RFC 2616, section 14.9.5.
Thanks to Tony Milllion for pointing out the HTTPS solution, and letting me know it affected O2 as well.