I experienced that when I submitted Bug 61018: The .odt was attached with mime type text/plain.
Solution. BSA should always switch to auto recognition.
Fixed in between? Worked fine with screenshot for Bug 61453 (.png) and a .odt in test Bug 61454
*** Bug 61845 has been marked as a duplicate of this bug. ***
Damned, I did not see the .odt relation. This bug today still exists, but probably the problem is not caused by BSA, but By Bugzilla due to "Bug 61845 - Attachment ODF document" recognized as "text/plain" by auto detect"
I submitted "Bug 62003 - BUGZILLAASSISTANT: Test Bug for odf attachments" to check whether auto mime type recognition still is broken, and yes, it is.