Allow querying for multiple MIME types in the allimages API

The 'aimime' parameter can now be a pipe-separated list of
MIME types. Also tweaked the English apihelp and added an
example for the new functionality.

Bug: T78690
Change-Id: I775ecae53c81eccb0a898081715d48589b53af15
Merge "Allow querying for multiple MIME types in the allimages API"