http://forums.zotero.org/discussion/6981/opening-bibtex-file-instead-of-importing/
Please note that the import progress bar does not disappear unless the browser is closed.
Defining import character encoding in settings (instead of auto-detect) appears to be a solution.
In the latest Zotero version (2.0b7.5) that was released today, this bug has been fixed. Auto detecting the encoding of the import file should work fine now – no need to select a specific import encoding.