Add "upload" type to API

If a file upload is not formatted correctly for PHP to recognize it as a
file upload rather than a regular field, the API will wind up trying to
load the file contents as a text field. Since these file contents are
often a large binary file, this will tend to run out of memory trying to
apply Unicode normalization.

To prevent this and to allow for a helpful error message, mark
parameters that are supposed to be file uploads.

Bug: 44909
Change-Id: Ia4586953e2ad2d72d08852689e060e39e7920d50