File: syntax as described at mw:Help:Images is inconsistent and does not work as expected in all cases. Linter fails to identify all problem cases. The "parameters" (for lack of a better word) and options used in the File: syntax do not work in a predictable, consistent way, and their implementation is inconsistent with how template parameters work.
See https://meta.wikimedia.org/wiki/Community_Wishlist_Survey_2021/Multimedia_and_Commons/Make_File_syntax_processing_match_the_specification_and_documentation for a full description and links to existing phabricator tasks
Programming skills, strong analytical skills to compare the specification with functionality, technical writing of documentation (mentor can help with documentation).
(A couple of these have been worked on a little in the past few months, but the whole system still needs a full evaluation.)
T216566: Capitalized versions of valid File options are usually (but not always) ignored, but are (usually but not always) not flagged as Linter bogus file options.
T216003: Linter fails to detect multiple "upright" parameters as a Bogus file option (there are multiple variants within this bug report, including inconsistent handling of spaces before and after equals signs in File: parameters. Note also that if there is a space character between alt and the equals sign, the alt statement will be treated as a caption.)
T215999: Linter does not detect invalid "500px500px" as a bogus file option (also "600 px", "left150px", duplicated "300px")
T179605: LintError bogus-image-options triggers on "Thumbtime" (in general, case sensitivity works inconsistently in File: syntax)
T266406: Linter false positive: "Center/Left/Right" as caption for gallery image (no workaround is available)
T264464: Conflicting border/thumb/frame options are not detected as Linter errors (Until October 2020, mw:Help:Images said "If multiple of these options are present, only the first one will be used". That statement was incorrect, in that some options have priority over others regardless of position, leading to inconsistent results for editors and readers.)
T276675: Image caption ending in "px" is incorrectly detected as a Linter error
T306216: Media options don't have the same whitespace leniency as template arguments