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The forums should allow all the file types that users might be posting about, and the 50 MB limit should be increased to at least 100 MB, preferrably 150 MB, to allow for raws from newer cameras and TIFFs, PSDs, and PSBs with typical dimensions.
As it stands now, even if you can explain to a user how to attach a file to a reply (I have a macro for that), chances are that it will fail due to an unsupported file type. And at that point, the user has lost patience and doesn't want to hear about uploading to Dropbox, etc. (which many users don't know how to do).
I've tested the common raw file types and other file types often referenced in the Lightroom forum:
File types allowed:
jpg, log, mov, mp4, psd, tif, txt, xmp
File types disallowed, with the error "The contents of the attachment doesn't [sic] match its file type.":
File types disallowed because the samples tested were larger than 50 MB:
File types disallowed, with the error 'The file ... does not have a valid extension":
The error message for .lua files implies that many of the file types that failed in my testing are intended to be supported (but note the error message is cut off, and good luck reading the low-res version created by the forum software):