What type of file is specifically used for filing internal angles and clearing corners?

The correct answer is a three-square file, which is specifically designed for filing internal angles and clearing corners. This type of file has three flat sides and a sharp edge, making it particularly effective for accessing tight spaces where other files might not fit. The unique shape of the three-square file enables it to reach into corners and provide precision in shaping and smoothing internal angles, which is a common requirement in various pipe fitting scenarios.

Other files, while useful for different applications, do not possess the same versatility in working on internal angles and corners. For example, square files are primarily used for making flat surfaces and have two flat surfaces that do not allow for the same degree of precision in corners as the three-square file. A mill file, on the other hand, is better suited for sharpening and smoothing flat surfaces rather than intricate internal angles. Lastly, a knife file has a single-edge design that is primarily used for detailing and finishing work but is not optimized for clearing corners as effectively as a three-square file. Thus, the three-square file stands out as the ideal choice for this specific task.