Update files.md to tag thumbnail code sample

[cbetta]

No description provided.

[boxcla]

Verified that @cbetta has signed the CLA. Thanks for the pull request!

[coveralls]

Pull Request Test Coverage Report for Build 1955

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 65.205%

---

Totals
Change from base Build 1954:	0.0%
Covered Lines:	5322
Relevant Lines:	8162

---

💛 - Coveralls

[carycheng]

Hey @cbetta, looks good, curious about the format - should it be thumbnail or thumbnail_id? Wondering what the id refers to in this case, is it's just arbitrary then no worries, thanks!

[cbetta]

Hey @carycheng, yes get_files_id_thumbnail_id is correct.

In general terms, we replace every variable in the URL with a generic id and replace every forward slash and dot into a _.

In this case, that means GET /files/:file_id/thumbnail.:extension becomes get_files_id_thumbnail_id. The last ID is the extension.

More on this here: https://confluence.inside-box.net/display/PLT/Sample+Tags