Fixes incompatibility issue with S3 Uploads plugin

[Genyus]

S3 Uploads plugin enables WordPress uploads to be offloaded to S3 hosting. When this plugin is activated, enabling WP Discourse connection logs throws an unhandled fatal error because the 'r+' mode is unsupported.

Read-write mode is unnecessary as the handle is only used for a single fstat() call, so regular read mode is sufficient. This change has no impact on standard configurations, but prevents the error when S3 is in use

Closes #458