Remove artificial restriction for file suffixes

[dharabor]

When uploading a trace, the app demands a specific kind of suffix. Suggest to remove this check and instead parse whatever input file the user specifies. If the input is malformed, throw a parsing error with additional information.

My reasoning is threefold:

(1) The restriction is artificial and creates additional doubt and friction for first-time users, who are probably just looking to work with output from a solver (e.g., piglet).

(2) Asking for specific suffixes makes sense for binary files (maybe) but less so for structured text files, where we can check every line of input.

(3) Throwing an error to say something is wrong gives us an opportunity to link the user to documentation sources, which explain valid input looks like.