The questions would be simple if they were based on a correct premise. As it is, simple yes or no answers would be misleading.
Phrases and slogans (although not necessarily what you describe) can be registered as trademarks.
BUT, you need to know that in the US trademarks (including slogans used as trademarks) are not created by registration, but by using the mark/slogan appropriately. What is appropriately? Use as a brand for selling goods or services -- that is, as a mark to show that goods/services bearing that mark all come from a common source -- and without causing a likelihood that customers will be confused between your mark and an earlier user of the same or a similar mark for similar goods or services.
Whether or not any mark can serve as (and/or be registered as) a trademark depends on a multifaceted analysis involving
1) the nature of the mark itself (e.g., Is it a generic term for the thing sold? Then no can do. Descriptive of the thing sold? It's weak as a mark but with use over time it may develop "secondary meaning" and be permitted to stand as a trademark. Inherently distinctive mark? Pretty much good to go unless you're encroaching on someone else's rights with it.) and
2) other factors including such things as the kinds of goods/services, the channels of commerce, and the sophistication of the consumers.
This is why anyone beginning a business and choosing a trademark would do well to hire a trademark attorney to do a proper trademark search and opinion on the viability of the proposed mark before investing in marketing. If that's out of the budget, then the business owner should read up as much as possible on what makes a strong trademark and what goes into an analysis of likelihood of confusion (the standard for infringement), then conduct a search of not only the list of registered trademarks, but also of trademarks that aren't registered (because their owners have rights, too) -- bearing in mind in the search that marks that sound similar or look similar or just have the same "commercial impression" can be problems, so you have to run many searches of various spellings of, and maybe even synonyms for, the key words.
As for whether a file extension can serve as a trademark, I'm not sure, but would expect the answer to depend on 1) whether the extension on a generated file can serve a branding function and, the related question, 2) whether precluding others from using the same file extension would violate antitrust laws by, in effect, using the trademark laws to unlawfully gain an unlimited monopoly over the underlying technology, which monopoly can only be granted (for limited periods of time) by patent or copyright on the software.