I found this issue while investing issue 0009890. We have three separate configuration values for selecting what file extensions can be viewed inline, shown as text, etc:

   # Extensions for text files that can be expanded inline.

$g_preview_text_extensions = array( 'txt', 'diff', 'patch' );

# Extensions for images that can be expanded inline.
$g_preview_image_extensions = array( 'bmp', 'png', 'gif', 'jpg', 'jpeg' );

# list of filetypes to view inline. This is a string of extentions separated by commas
# This is used when downloading an attachment.  Rather than downloading, the attachment
# is viewed in the browser.
$g_inline_file_exts = 'gif,png';</pre>
It seems to me that:

The duplication of image extensions is confusing and unnecessary
The difference in definition (array vs string) is also confusing

To add to all this, when viewing the list of bug attachments, the following configuration setting is improperly checked:
   # Specifies the maximum size (in bytes) below which an attachment is 

previewed in the bug view pages.
# To disable this feature, set max size to 0.
$g_preview_attachments_inline_max_size = 0;</pre>
When listing attachments, the code only checks that 'file_size <= max_size', and ignores the documented (and default) use of 0 as meaning 'no limit'.  This should be rectified in core/file_api.php (1.1.x) and core/print_api.php (1.2.x).