Scenario/Problem: You have a file with an extension, but the Finder says it cannot open the file, and you have no idea what kind of file it is. How do you determine the file type and its associated software?

Solution: After determining the file’s extension, you should look up the extension in an online database. Here are a few suggested sources.

How to Identify Unknown File Type

• FileInfo: This site has a very large database of extensions. You can search by extension or browse by file type. The search results will describe the type of file and associated applications. What is nice about this site is that many file types include Mac applications, whereas most other sites include only Windows applications. Note: If a file extension page does not list any compatible Mac applications, that does not preclude such a possibility. It simply means the web site does not know if there are compatible applications.
• File-Extensions: This site is very similar to FileInfo, but there are two noteworthy differences. First, the file extension descriptions generally do not list Mac applications. Second, it is common for file extensions to have not only more than one application associated with it but also more than one type of file associated with. For example, while .asc is typically an ASCII text file that any old text editor can open, it can also be a 3D Studio Max scene file, which means that it could be a configuration file for a 3D movie renderer. Obviously, those are very different types of files. File-Extensions does a much better job than FileInfo of showing you all the possible file types – not just the most common ones.

Once you have used one of these resources to determine what kind of file you have, you will have to figure out which software can read the file. If FileInfo indicated that a Mac OS X application will open the file, and you already have that application, then you can try opening the file with that software. Otherwise, here are some further troubleshooting steps.

Chose software to open the files

1. If you have already been able to determine which Mac OS X software you need, then you simply need to acquire the software. If it is commercial software, you will need to purchase it online or in a store. You might, however, visit the software manufacturer’s website to see if it offers a free demo version of the software. This will allow you to verify whether the software can really open your file. You could also visit AlternativeTo to see if there is a more cost-effective solution.
2. If the software is freeware/shareware, you likely can use MacUpdate or Version Tracker to find the software or do a Google search to find the author’s website.
3. If you do not wish to purchase expensive software, or if FileInfo indicated any number of applications may be able to read the file, then it may be worth doing a Google search to see what other Mac users are doing to open these files. For example, if you’re trying to open a QIF file, you might search for “Mac OS X open QIF files.” You can visit Apple’s Discussions forums and search there, as well. – discussions.apple.com
4. If neither FileInfo nor File-Extensions indicate there is a Mac-compatible application for the file, you should do a Google search, as well. Some files simply cannot be opened (or done so without much effort). For example, if you receive a Microsoft Publisher file (which may have a .pub extension), there is currently no Mac application that can read the file. You might still run a search, such as “Mac OS X how to open Microsoft Publisher files.” If the end result is that you will need to open the file in Publisher on Windows, at least you will know.