Comment 1 for bug 401772

Investigation suggests that Gmail uses the filename and content-type to determine whether to display the attachment inline.

If the file extension is '.txt', or the content-type is 'text/plain', the message is displayed inline. (It's not known which extensions are acceptable, but .patch and .diff have been determined unacceptable.)

We could override the content-type or file extension for gmail.com addresses, but that would not reach all gmail users, because some people use gmail to read mail for other domains.

If we try to use one message type for both gmail and conforming clients, it seems best to retain the content-type, since conforming clients will respect it, and set the filename such that it pleases gmail.

Finally, even when patches are displayed inline, they are displayed in a variable-width font, so they are not very readable. However, Gmail provides a "View" link that displays them in a fixed-width font.