fix #95: Image Attachment Disappears After Audio Recording

[csells]

Fixes the case that an audio recording clears out the existing attachments, if there are any.

issues fixed by this PR:

• Image Attachment Disappears After Audio Recording #95: Image Attachment Disappears After Audio Recording

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.