$file_array variable is never assigned as an array in media_sideload_image()

[danielbachhuber]

In media_sideload_image(), the $file_array variable isn't created as an array. It should be, to ensure it never inherits values from a similarly named variable in later refactoring.

[SergeyBiryukov]

In 29215:

Declare the variable before using it.

props danielbachhuber.
fixes #28932.