With regard to What Are Attachments?
Below is a list of the current allowed Forum file attachment types and their corresponding maximum file size...
Extension -> Max Filesize (Bytes) -> Max Width -> Max Height
3ds -> 2097152
bmp -> 524288 -> 800 -> 600
ctb -> 104858
cui -> 1048576
dcl -> 104858
doc -> 1048576
docx -> 1048576
dvb -> 1048576
dwf -> 1048576
dwg -> 2097152
dwt -> 524288
fbx -> 2097152
gif -> 524288 -> 800 -> 600
jpe -> 1048576 -> None -> None
jpeg -> 1048576 -> None -> None
jpg -> 1048576 -> None -> None
lin -> 104858
lsp -> 104858
ma -> 2097152
mat -> 2097152
max -> 2097152
mb -> 2097152
mns -> 104858
mnu -> 104858
pat -> 104858
pdf -> 1048576
pgp -> 104858
png -> 1048576 -> None -> None
psd -> 524288 -> None -> None
rfa -> 2097152
rft -> 2097152
rte -> 2097152
rvt -> 2097152
shp -> 524288
shx -> 524288
stb -> 104858
txt -> 104858
xls -> 1048576
xlsx -> 1048576
zip -> 15728640